EG&G ORTEC 572 Service manual

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Model 572
Spectroscopy Amplifier
and
Pile-Up Rejector
Operating and Service Manual
Mm

Fast Coincidence
The ORTEC 414A Fast Coincidence is a
modular threefold coincidence unit that
allows fast coincidence determination be
tween any two or three input signals. The
term "fast" indicates the general nature
of the coincidence circuit: i .e., input
pulses are reshaped and the actual coinci
dence determination is made on the
leading edge, or leading portion, of the
pulses. A dc-coupled anticoincidence in
put is provided to inhibit the coincidence
output by a dc voltage or a pulse that
overlaps the period of coincidence of the
coincident pulses. The coincidence inputs
are ac-coupled and all four inputs are
controlled by In/Out toggle switches.
The resolving time (2t) of the Fast
Coincidence unit may be varied over a
10- to 110-nsec range by a 10-turn
control for accurate resettabil ity of the
resolving time. The resolving time of
the anticoincidence circuit is set by the
width of the input pulse.
The 414A is one of the ORTEC 400
Series modular nuclear instruments and
conforms to the standards of TID-20893
(Rev.). An ORTEC 401/402 Series Bin
and Power Supply provides all necessary
power through a rear module connector.
Outputs from other related ORTEC tim
ing equipment are compatible with the
input requirements of the 414A.
SPECIFICATIONS
PERFORMANCE
PULSE PAIR RESOLUTION <100 nsec on
any single input: for coincidence events, <1
Msec on the coincidence output.
RESOLVING TIME (2t) Continuously variable
from 10 to 110 nsec for coincidence signals;
set by the width of the input pulse for the
anticoincidence signal.
TEMPERATURE INSTABILITY
<0.2%/°C, Oto 50°C.
CONTROLS
2t changes
RESOLVING TIME, 10-110 nsec Front panel
10-turn potentiometer for centrolling resolving
time for inputs A, B, and C over a range of
10 to 110 nsec.
INPUT CONTROLS Toggle switches for using
any input combination desired and fordisabling
input signals to the coincidence and anti
coincidence circuits without input coaxial
cables having to be removed.
INPUTS
GOING Front panel BNC connectors provide
3 ac-coupled coincidence inputs (A,B,C) of
positive polarity; 2-V threshold, 20-nsec mini
mum width required; absolute maximum input
50 V; impedance >3000n.
ANTIGOING Front panel BNC connector
provides one dc-coupled anticoincidence input
(D) for inhibiting coincidence output; -i-2-V
threshold, 20-nsec minimum width required;
absolute max input 50 V; impedance >300012.
OUTPUTS
OUTPUTS 2 separate buffered coincidence
output signals through BNC connectors on front
panel provide positive pulses 500 nsec wide
with 5-V minimum amplitude; ac-coupled with
<1022 impedance; monitored through oscillo
scope test points on front panel.
ORDERING INFORMATION
POWER REQUIRED
+24 V, 30 mA; -24 V, 30 mA;
+ 12 V, 120 mA; -12 V, 85 mA.
WEIGHT (Shipping) 4.4 lb (2.0 kg).
WEIGHT (Net) 2.4 lb (1.09 kg).
DIMENSIONS NIM-standard double-width
module (2.70 by 8.714 in.) per TID-20893
(Rev.).
•Three coincidence inputs
•Adjustable 10- to 110-nsec
resolving time
•Anticoincidence input dc-coupled
FAST COINOOENCE
Printed in U.S.A.
1196 1178
n
P' Offices in 49 Countries. For more information on EG&G ORTEC products or their applications, contact your local representative or:
^ United States: EG&G ORTEC. 100 Midland Road. Oak Ridge. TN 37830, telephone [phone]. Telex 55-7450: W. Germany: EG&G ORTEC GmbH. Munich, tele-
phone [phone]; United Kingdom: EG&G ORTEC Limited. Bracknell. telephone 55455: France: EG&G ORTEC S A R L., Rungis Cedex, telephone 687-
\ I 25-71; Italy: EG&G ORTEC SpA, Milan, telephone [phone]; Brazil: EG&G ORTEC Limitada, Sao Paulo, S.P , telephone [phone]

EG5G ORTEC MODEL 572 AMPLIFIER
Manual Change Sheet
February 3, 1981
ECN #28
On schematic 572-0201-Sl at the back of this manual, please make the
following changes;
1. Change the description of A1 HOA0201 to read
A1 HOA0201 or HGA0401.
2. Change the descriptions of A2 HOAOlOl and A3 HOAOlOl to read
A2 HOAOlOl or HOA0301 and A3 HOAOlOl or HOA0301.
Model 572
Spectroscopy Amplifier
and
Pile-Up Rejector
Operating and Service Manual
This manual applies to instruments marked
"Rev 27" on rear panel
Printed in U.S.A. 2961 7.50 1180

Ill
CONTENTS
WARRANTY
PHOTOGRAPHS
1. DESCRIPTION
1.1. General
1.2. Pole-Zero Cancellation
1.3. Active Filter
Page
V
vi
1
1
1
3
2. SPECIFICATIONS
2.1. Performance
2.2. Controls
2.3. Input
2.4. Outputs
2.5. Electrical and Mechanical
3. INSTALLATION
3.1. General
3.2. Connection to Power
3.3. Connection to Preamplifier
3.4. Connection of Test Pulse Generator
3.5. Shaping Considerations
3.6. Use of Delayed Output
3.7. Linear Output Connections and Terminating Considerations
3.8. Shorting or Overloading the Amplifier Outputs
3.9. Inhibit Output Connection
3.10. Busy Output Connection
3.11. CRM Output Connection
4
4
4
4
4
5
5
5
5
5
5
6
6
6
4. OPERATION
4.1. Initial Testing and Observation of Pulse Waveforms
4.2. Front Panel Controls
4.3. Front Panel Connectors
4.4. Rear Panel Connectors 8
4.5. Standard Setup Procedure 8
4.6. Pole-Zero Adjustment 9
4.7. BLR Threshold Adjustment 11
4.8. Operation with Semiconductor Detectors 11
4.9. Operation in Spectroscopy Systems 13
4.10. Other Experiments 14
5. CIRCUIT DESCRIPTION 17
6. MAINTENANCE 19
6.1. Test Equipment Required 19
6.2. Pulser Test 19
6.3. Suggestions for Troubleshooting 21
6.4. Factory Repair 21
6.5. Tabulated Test Point Voltages 21
Schematic
572-0201-SI

IV
ILLUSTRATIONS
Fig. 1.1. Differentiation in an Amplifier Without Pole-Zero Cancellation 2
Fig. 1.2. Differentiation in a Pole-Zero-Cancelled Amplifier 2
Fig. 1.3. Pulse Shapes for Good Signal-to-Noise Ratios 3
Fig. 4.1. Typical Effects of Shaping-Time Selection on Output Waveforms 8
Fig. 4.2. Typical Waveforms Illustrating Pole-Zero Adjustment Effects; Oscilloscope 9
Fig. 4.3. A Clamp Circuit that Can Be Used to Prevent Overloading the Oscilloscope Input . . . . 10
Fig. 4.4. Pole-Zero Adjustment Using a Square Wave Input to the Preamplifier 10
Fig. 4.5. BLR Threshold Variable Control Settings 11
Fig. 4.6. System for Measuring Amplifier and Detector Noise Resolution 12
Fig. 4.7. Noise as a Function of Bias Voltage 12
Fig. 4.8. System for Measuring Resolution with a Pulse Height Analyzer 12
Fig. 4.9. System for Detector Current and Voltage Measurements 13
Fig. 4.10. Silicon Detector Back Current vs Bias Voltage 13
Fig. 4.11. System for High-Resolution Alpha-Particle Spectroscopy 14
Fig. 4.12. System for High-Resolution Gamma Spectroscopy 14
Fig. 4.13. Scintillation-Counter Gamma Spectroscopy System 14
Fig. 4.14. High-Resolution X-Ray Energy Analysis System Using a Proportional Counter 15
Fig. 4.15. General System Arrangement for Gating Control 15
Fig. 4.16. Gamma-Ray Charged-Particle Coincidence Experiment 16
Fig. 4.17. Gamma-Ray Pair Spectrometry 16
Fig. 4.18. Gamma-Gamma Coincidence Experiment 17
Fig. 5.1. Amplifier Block Diagram 18
Fig. 5.2. Timing Relations in the 572 Amplifier and Pile-Up Rejector 19
Fig. 6.1. Circuit Used to Measure Nonlinearity 20

STANDARD WARRANTY FOR EG&G ORTEC INSTRUMENTS
EG&G ORTEC warrants that the items will be delivered free from defects in material or workmanship. EG&G ORTEC
makes no other warranties, express or implied, and specifically NO WARRANTY OF MERCHANTABILITY OR FITNESS
FOR A PARTICULAR PURPOSE.
EG&G ORTEC's exclusive liability is limited to repairing or replacing at EG&G ORTEC's option, items found by EG&G
ORTEC to be defective in workmanship or materials within one year from the date of delivery. EG&G ORTEC's liability on
any claim of any kind, including negligence, loss or damages arising out of, connected with, or from the performance or
breach thereof, or from the manufacture, sale, delivery, resale, repair, or use of any item or services covered by this agree
ment or purchase order, shall in no case exceed the price allocable to the item or service furnished or any part thereof
that gives rise to the claim. In the event EG&G ORTEC fails to manufacture or deliver items called for in this agreement or
purchase order, EG&G ORTEC's exclusive liability and buyer's exclusive remedy shall be release of the buyer from the
obligation to pay the purchase price. In no event shall EG&G ORTEC be liable for special or consequential damages.
QUALITY CONTROL
Before being approved for shipment, each EG&G ORTEC instrument must pass a stringent set of quality control tests
designed to expose any flaws in materials or workmanship. Permanent records of these tests are maintained for use in
warranty repair and as a source of statistical information for design improvements.
REPAIR SERVICE
If it becomes necessary to return this instrument for repair, it is essential that Customer Services be contacted in advance
of its return so that a Return Authorization Number can be assigned to the unit. Also, EG&G ORTEC must be informed,
either in writing or by telephone [[phone]], of the nature of the fault of the instrument being returned and of the
model, serial, and revision ("Rev" on rear panel) numbers. Failure to do so may cause unnecessary delays in getting the
unit repaired. The EG&G ORTEC standard procedure requires that instruments returned for repair pass the same quality
control tests that are used for new-production instruments. Instruments that are returned should be packed so that they
will withstand normal transit handling and must be shipped PREPAID via Air Parcel Post or United Parcel Service to the
nearest EG&G ORTEC repair center. The address label and the package should include the Return Authorization Number
assigned. Instruments being returned that are damaged in transit due to inadequate packing will be repaired at the
sender's expense, and it will be the sender's responsibility to make claim with the shipper. Instruments not in warranty
will be repaired at the standard charge unless they have been grossly misused or mishandled, in which case the user will
be notified prior to the repair being done. A quotation will be sent with the notification.
DAMAGE IN TRANSIT
Shipments should be examined immediately upon receipt for evidence of external or concealed damage. The carrier
making delivery should be notified immediately of any such damage, since the carrier is normally liable for damage in
shipment. Packing materials, waybills, and other such documentation should be preserved in order to establish claims.
After such notification to the carrier, please notify EG&G ORTEC of the circumstances so that assistance can be provided
in making damage claims and in providing replacement equipment if necessary.

VI
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ORTEC 572 SPECTROSCOPY AMPLIFIER
AND PILE-UP REJECTOR
1. DESCRIPTION
1.1. GENERAL
The ORTEC 572 Spectroscopy Amplifier and Pile-Up
Rejector is a single-width NIM module with a versatile
combination of switch-selectable pulse-shaping and out
put characteristics. It features extremely low noise, wide
gain range, and excellent overload response for universal
application in high-resolution spectroscopy. It accepts
input pulses of either polarity that originate in germanium
or silicon semiconductor detectors, in scintillation
counters with either fast or slow scintillators, in propor
tional counters, in pulsed ionization chambers, in elec
tron multipliers, etc.
The 572 has an input impedance of approximately 500fl
and accepts either positive or negative input pulses with
rise times <650 ns and fall times >40 fis. Six integrate and
differentiate time constants are switch-selectable to pro
vide optimum shaping for resolution and count rate. The
first differentiation network has variable pole-zero can
cellation that can be adjusted to match preamplifiers with
decay times >40 ^xs. The pole-zero cancellation dras
tically reduces the undershoot after the first differentiator
and greatly improves overload and count rate charac
teristics. In addition, the amplifier contains an active filter
shaping network that optimizes the signal-to-noise ratio
and minimizes the overall resolving time. Both unipolar
and bipolar outputs are provided simultaneously on the
front and rear panels.
The unipolar output should be used for spectroscopy
when dc coupling can be maintained from the 572 to the
analyzer. A BLR (baseline restorer) circuit is included in
the 572 for improved performance at all count rates.
Baseline correction is applied during intervals between
input pulses only and a front panel switch selects a
discriminator level to identify input pulses. The unipolar
output dc level can be adjusted in the range from -lOOmV
to +100 mV. This output permits the use of the direct-
coupled input of the analyzer with a minimum amount of
interface problems. The 572 bipolar output may be
preferable for spectroscopy when operating into an ac-
coupled system at high counting rates.
Internal pulse pileup (a second pulse arriving before the
first pulse has been completed) is sensed internally. The
572 includes an Inhibit output BNC connector on the rear
panel that can be used to inhibit measurement of the
result of a pulse pileup when it occurs.
The 572 can be used for constant-fraction timing when
operated in conjunction with an ORTEC 551, 552, or 553
Timing Single-Channel Analyzer. The ORTEC Timing
Single-Channel Analyzers feature a minimum of walk as a
function of pulse amplitude and incorporate a variable
delay time on the output pulse to enable the timing pickoff
output to be placed in time coincidence with other
signals. A switch-selectable 2-ns delay is also provided on
the unipolar output to aid in obtaining the proper timing of
the linear pulse in a coincidence-gated system.
The 572 has complete provisions, including power, for
operating any ORTEC solid-state preamplifier. Normally,
the preamplifier pulses should have a rise time of 0.25 yus
or less to properly match the amplifier filter network and a
decay time greater than 40 ns for proper pole-zero
cancellation. The 572 input impedance is 500n. When
long preamplifier cables are used, the cables can be
terminated in series at the preamplifier end or in shunt at
the amplifier end with the proper resistors. The output
impedance is about 0.1 fl at the front panel connectors
and 93fl at the rear panel connectors. The front panel
outputs can be connected to other equipment by a single
cable going to all equipment and shunt terminated at the
far end of the cabling. If series termination is desired, the
rear panel connectors can be used to connect the 572 to
other modules. See Section 3 for further information.
1.2. POLE-ZERO CANCELLATION
Pole-zero cancellation is a method for eliminating pulse
undershoot after the first differentiating network. In an
amplifier not using pole-zero cancellation (Fig. 1.1) the
exponential tail on the preamplifier output signal (usually
50 to 500 /us) causes an undershoot whose peak amplitude
is roughly determined from:
undershoot amplitude
differentiated pulse amplitude
differentiation time
preamplifier pulse decay time
For a ^-^lS differentiation time and a 50-mS pulse decay
time the maximum undershoot is 2%, and this decays with
a 50-^xs time constant. Under overload conditions this
undershoot is often sufficiently large to saturate the
amplifier during a considerable portion of the under
shoot, causing excessive dead time. This effect can be
reduced by increasing the preamplifier pulse decay time
(which generally reduces the counting rate capabilities of
the preamplifier) or compensating for the undershoot by
using pole-zero cancellation.
Pole-zero cancellation is accomplished by the network
shown in Fig. 1.2. The pole [s + (1/To)] due to the
preamplifier pulse decay time is cancelled by the zero of

Bo (t)
o(t) = Ernax
I
e, (t)
Undershoot
nk^r^ First Differentiated
olJtnSt X differentiation = pulse
output A
undersfioot
^max X G(t) = e,(t).
1 s
X = Ei(s) (Laplace transform).
■ max 1 1
s + — s +
7" o Rj Ci
200409
-r To e-f/^o _ 7", e-f/7"o = g, (t), where r, = R, C,
10 - ' \
Fig. 1.1. Differentiation in an Ampiifier Without Poie-Zero Canceilation.
0 <Ar<i
Bo (t) ^rnax b ^
Bi (t)
No Undershoot
Pole-zero Differentiated
Charge loop w cancelled _ pulse
output differentiation without
network undershoot
Pole zero cancel by letting
1 K
s + — = s + ■
T o RzCi
£"max X G(t) =e,(t) .
s +
_!_, Rzc.
k. mav .• ^
"lax 1 " R , + Rj
'""To ^""R.RzC,
E\ (s), (Laplace transform).
-max r^max
RI -r Rt
s + _ _ _ s +
R
:j = £ I (sj, where Rp =
R.Rz
R, + Rj
I RzC] RpCi
p Q f/Rp^l - P, /ti
r^max " - ci ii; .
200410
Fig. 1.2. Differentiation in a Pole-Zero Canceiied Amplifier.

the network [s + (K/R2C1)]. In effect, the do path across
the differentiation capacitor adds an attenuated replica of
the preamplifier pulse to just cancel the negative under
shoot of the differentiating network.
Total preamplifier-amplifier pole-zero cancellation re
quires that the preamplifier output pulse decay time be a
single exponential decay and matched to the pole-zero
cancellation network. The variable pole-zero cancellation
network allows accurate cancellation for all preamplifiers
having 40-^8 or greater decay times. Improper matching
of the pole-zero network will degrade the overload perfor
mance and cause excessive pileup distortion at medium
counting rates. Improper matching causes either an
undercompensation (undershoot is not eliminated) or
an overcompensation (output after the main pulse does
not return to the baseline but decays to the baseline with
the preamplifier time constant). The pole-zero adjust is
accessible on the front panel of the 572 and can easily be
adjusted by observing the baseline on an oscilloscope
with a monoenergetic source or pulser having the same
decay time as the preamplifier under overload conditions.
The adjustment should be made so that the pulse returns
to the baseline in the minimum time with no undershoot.
1.3. ACTIVE FILTER
When only FET gate current and drain thermal noise are
considered, the best signal-to-noise ratio occurs when
the two noise contributions are equal for a given input
pulse shape. The Gaussian pulse shape (Fig. 1.3) for this
condition requires an amplifier with a single RC differen
tiate and n equal RC integrates where n approaches in
finity. The Laplace transform of this transfer function is
G(s) =
1
s + (1/RC) [s + (1/RC)]"
(n - =0).
where the first factor is the single differentiate and the
second factor is the n integrates. The 572 active filter
approximates this transfer function.
Figure 1.3 illustrates the results of pulse shaping in an
amplifier. Of the four pulse shapes shown the cusp would
produce minimum noise but is impractical to achieve with
normal electronic circuitry and would be difficult to
measure with an ADC. The true Gaussian shape
deteriorates the signal-to-noise ratio by only about 12%
from that of the cusp and produces a signal that is easy to
measure, but requires many sections of integration (n
— 00). With two sections of integration the waveform
identified as a Gaussian approximation can be obtained,
and this deteriorates the signal-to-noise ratio by about
22%. The ORTEC active filter network in the 572 Amplifier
provides the fourth waveform in Fig. 1.3; this waveform
has characteristics superior to the Gaussian approxima
tion, yet obtains them with four complex poles. By this
method the output pulse shape has a good signal-to-
noise ratio, is easy to measure, and yet requires only a
Ralative
Noise
CUSP
I/RC
GAUSSIAN
1.12
1
s + (imC) ls + (1/RCIl"
1.22
GAUSSIAN APPROXIMATION
FOR n - 2
(1/RCI U + (1/RC))'
ACTIVE FILTER
-1.17
(s + (1/RC)1
1
[s + (3/RC)l [s + (1 - kj)/RCl Is + (1 + kj)/RCl
j = k = 0.3
Fig. 1.3. Puise Shapes for Good Signal-to-Noise Ratios.
practical amount of electronic circuitry to achieve the
desired results.

2. SPECIFICATIONS
2.1. PERFORMANCE
GAIN RANGE Continuously adjustable from XI
through X1500.
PULSE SHAPING Gaussian on all ranges with peaking
time equal to 2.2r and pulse width at 0.1% level equal to 2.9
times the peaking time.
INTEGRAL NONLINEARITY <0.05% (0.025% typical)
using 2 ^is shaping.
NOISE <8 /jV referred to the input (5 typical) using 2
MS shaping and gain ^100.
TEMPERATURE INSTABILITY
Gain <0.0075%/° 0, 0 to 50° C.
DC Level <±50 mV/°G, 0 to 50°C.
CROSSOVER WALK <±3 ns for 50:1 dynamic range,
including contribution of ORTEG 551 or 552 Constant-
Fraction Timing Single-Channel Analyzer using 50%
fraction and 0.5 fxs shaping.
COUNT RATE STABILITY The 1.33 MeV gamma ray
peak from a "'Co source, positioned at 85% of analyzer
range, typically shifts <0.024%, and its FWHM broadens
<16% when its incoming count rate changes from 0 to
100 000 counts/s using 2 ms shaping. The amplifier will
hold the baseline reference up to count rates in excess
of 150 000 counts/s.
OVERLOAD RECOVERY Recovers to within 2% of rated
output from X300 overload in 2.5 nonoverloaded unipolar
pulse widths, using maximum gain; same recovery from
XI000 overload for bipolar pulses.
2.2. CONTROLS
GAIN Ten-turn precision potentiometer for continu
ously variable direct-reading gain factor of X0.5 to XI.5.
COARSE GAIN Six-position selector switch selects
feedback resistors for gain factors of 20, 50,100,200,500,
and IK.
INPUT ATTENUATOR Jumper on printed circuit board
selects an input attenuation factor of 1 or 10 (gain factor of
XI or X0.1).
POS/NEG Toggle switch selects input circuit for either
polarity of input pulses from the preamplifier.
SHAPING TIME Six-position switch selects time con
stant for active filter network pulse shaping; selections are
0.5, 1, 2, 3, 6, and 10 ms.
PZ ADJ Potentiometer to adjust pole-zero cancellation
for decay times from 40 tis to °o. Factory preset at 50 ns to
match normal characteristics of ORTEC preamplifiers.
BLR Toggle switch selects a source for the gated
baseline restorer discriminator threshold level from one
of three positions.
Auto The BLR threshold is automatically set to an
optimum level as a function of the signal noise level by
an internal circuit. This allows easy setup and very good
performance under most conditions.
PZ Ad] The BLR threshold is determined by the thresh
old potentiometer. The BLR time constant is greatly In
creased to facilitate PZ adjustment. This position may
give the lowest noise for conditions of low count rate
and/or longer shaping times.
Threshold The BLR threshold Is set manually by the
threshold potentiometer. Range is 0 to 300 mV referred to
the positive output signal. The BLR time constant is the
same as for the Auto switch setting.
DC Screwdriver potentiometer adjusts the unipolar
output baseline dc level; range, +100 mV to -100 mV.
DELAY/OUT Toggle switch selects either 2 ms delay
(Delay) or prompt (Out) timing for the unipolar output.
2.3. INPUT
INPUT Type BNC front and rear panel connectors
accept either positive or negative pulses with rise times in
the range from 10 to 650 ns and decay times from 40 to
2000 fis: Z,„ ~500n, dc coupled; linear maximum, 1 V (10 V
with attenuator jumper set at X0.1); absolute maximum,
20 V.
2.4. OUTPUTS
UNI Unipolar front panel BNC with Z„ <111 and rear
panel BNC with Z„ = 93n. Short-circuit proof; prompt or
delayed, full scale linear range 0 to +10 V; active filter
shaped and dc restored; dc level adjustable to ±100 mV.
Bl Bipolar front panel BNC with Z„ <1 n and rear panel
BNC with Z„ = 9311. Short-circuit proof; prompt output
with positive lobe leading and linear range of ±10 V; active
filter shaped.
BUSY Rear panel BNC with Z„ <10n provides a +5 V
logic pulse for the duration that the input pulse exceeds
the baseline restorer discriminator level. Connect to the
ORTEC MCA Busy input for dead time correction.
INH Inhibit rear panel BNC with Z„ <10n provides a
nominal +5 V logic signal when an internal pulse pileup
occurs; width ~6r in coincidence with the pileup; to be
used for an MCA anticoincidence input to prevent storage
of pileup data in the spectrum.
CRM Count Ratemeter rear panel BNC furnishes a
nominal +5 V logic signal for every linear input pulse;
width, 500 ns; to be used as an input to a ratemeter or
counter.
PREAMP POWER Rear panel standard ORTEC power
connector; Amphenol 17-10090; mates with captive and
non-captive power cords on all standard ORTEC
preamplifiers.

2.5. ELECTRICAL AND MECHANICAL
POWER REQUIRED (not Including any load on the
Preamp Power connector)
+24 V, 100 mA; -24 V, 105 mA;
+12 V, 85 mA;-12 V, 50 mA.
FRONT PANEL DIMENSIONS NIM-standard single-
width module (1.35 by 8.714 in.) per TID-20893.
3. INSTALLATION
3.1. GENERAL
The 572 operates on power that must be furnished from a
NIM-standard bin and power supply such as the ORTEC
401/402 Series. The bin and power supply is designed for
relay rack mounting. If the equipment is to be rack
mounted, be sure that there is adequate ventilation to
prevent any localized heating of the components that are
used in the 572. The temperature of equipment mounted
in racks can easily exceed the maximum limit of 50°C
unless precautions are taken.
3.2. CONNECTION TO POWER
The 572 contains no internal power supply and must
obtain the necessary dc operating power from the bin and
power supply in which it is installed for operation. Always
turn off power for the power supply before inserting or
removing any modules. After all modules have been
installed in the bin and any preamplifiers have also been
connected to the Preamp Power connectors on the
amplifiers, check the dc voltage levels from the power
supply to see that they are not overloaded. The ORTEC
401/402 Series Bins and Power Supplies have convenient
test points on the power supply control panel to permit
monitoring these dc levels. If any one or more of the dc
levels indicates an overload, some of the modules will
need to be moved to another bin to achieve operation.
3.3. CONNECTION TO PREAMPLIFIER
The preamplifier output signal is connected to the 572
through the appropriate Input BNG connector on the
front or rear panel. The input impedance is about 500il
and is dc-coupled to ground; therefore the preamplifier
output must be either ac-coupled or have approximately
zero dc voltage under no-signal conditions.
The 572 incorporates pole-zero cancellation in order to
enhance the overload and count rate characteristics of
the amplifier. This technique requires matching the
network to the preamplifier decay time constant in order
to achieve perfect compensation. The pole-zero
adjustment should be set each time the preamplifier or the
shaping time constant of the amplifier is changed. For
details of the pole-zero adjustment see Section 4.6. An
alternate method is accomplished easily by using a
monoenergetic source and observing the amplifier
baseline with an oscilloscope after each pulse under
approximately X2 overload conditions. Adjustment
should be made so that the pulse returns to the baseline in
a minimum amount of time with no undershoot.
Preamplifier power at +24 V, -24 V, +12 V, and -12 V is
available through the Preamp Power connector on the
rear panel. When the preamplifier is connected, its power
requirements are obtained from the same bin and power
supply as is used for the amplifier, and this increases the
dc loading on each voltage level over and above the
requirements for the 572 at the module position in the bin.
When the 572 is used with a remotely located preamplifier
(i.e., preamplifier-to-amplifier connection through 25 ft or
more of coaxial cable), be careful to ensure that the
characteristic impedance of the transmission line from
the preamplifier output to the 572 input is matched. Since
the input impedance of the 572 is about 500il, sending-
end termination wil l normally be preferred; the
transmission line should be series-terminated at the
preamplifier output. All ORTEC preamplifiers contain
series terminations that are either 93fl or variable; coaxial
cable type RG-62/U or RG-71/U is recommended.
3.4. CONNECTION OF TEST
PULSE GENERATOR
THROUGH A PREAMPLIFIER The satisfactory
connection of a test pulse generator such as the ORTEC
419 Precision Pulse Generator or equivalent depends
primarily on two considerations; the preamplifier must be
properly connected to the 572 as discussed in Section 3.3,
and the proper input signal simulation must be applied to
the preamplifier. To ensure proper input signal simula
tion, refer to the instruction manual for the particular pre
amplifier being used.
DIRECTLY INTO THE 572 Since the input of the 572 has
500(1 of input impedance, the test pulse generator will
normally have to be terminated at the amplifier input with
an additional shunt resistor. In addition, if the test pulse

generator has a dc offset, a large series Isolating capacitor
is also required since the 572 input is dc coupled. The
ORTEC test pulse generators are designed for direct
connection. When any one of these units is used, it should
be terminated with a lOOn terminator at the amplifier
input or be used with at least one of the output attenuators
set at In. (Thesmall errorduetothefinite input impedance
of the amplifier can normally be neglected.)
SPECIAL CONSIDERATIONS FOR POLE-ZERO
CANCELLATION When a tail pulser is connected
directly to the amplifier input, the PZ Adj should be
adjusted if overload tests are to be made (other tests are
not affected). See Section 4.6 for the pole-zero
adjustment. If a preamplifier is used and a tail pulser is
connected to the preampl ifier test input, similar
precautions are necessary. In this case the effect of the
pulser decay must be removed; i.e., astep input should be
simulated.
3.5. SHAPING CONSIDERATIONS
The shaping time constant on the 572 is switch-selectable
in steps of 0.5, 1, 2, 3, 6, and 10 ns. The choice of the
proper shaping time constant is generally a compromise
between operating at a shorter time constant for
accommodation of high counting rates and operating
with a longer time constant for a better signal-to-noise
ratio. For scintillation counters the energy resolution
depends largely on the scintillator and photomultiplier,
and therefore a shaping time constant of about fourtimes
the decay-time constant of the scintillator is a reasonable
choice (for Nal, a 1-;us shaping time constant is about
optimum). For gas proportional counters the collection
time is normally in the 0.5 to 5 /us range and a 2 /is or
greater time constant selection will generally give
optimum resolution. For surface barrier semiconductor
detectors, a 0.5 to 2 resolving time will generally
provide optimum resolution. Shaping time for Ge(Li)
detectors will vary from 1 to 6 /us, depending on the size,
configuration, and collection time of the specific detector
and preamplifier. When a charge-sensitive preamplifier is
used, the optimum shaping time constant to minimize the
noise of a system can be determined by measuring the
output noise of the system and dividing it by the system
gain. Since the 572 has almost constant gain for all
shaping modes, the optimum shaping can be determined
by measuring the output noise of the 572 with a voltmeter
as each shaping time constant is selected.
The 572 provides both unipolar and bipolar outputs. The
unipolar output pulses should be used in applications
where the best signal-to-noise ratio (resolution) is most
important, such as high-resolution spectroscopy using
semiconductor detectors. Use of the unipolar output with
baseline restoration will also give excellent resolution at
high counting rates. The bipolar output should be used in
high count rate systems when the analyzer system is ac-
coupled and noise, or resolution, is a secondary
consideration.
3.6. USE OF DELAYED OUTPUT
The unipolar output can be either prompt or delayed. The
prompt output is used for normal spectroscopy applica
tions. The delayed output (equal in amplitude to the
prompt output but delayed by 2 ;^s) is used in coincidence
experiments where the delay can compensate for time
delays in obtaining the coincidence information. The
considerations regarding the proper choice of shaping
time for the delayed output are discussed in Section 3.5.
3.7. LINEAR OUTPUT CONNECTIONS AND
TERMINATING CONSIDERATIONS
Since the 572 unipolar output is normally used for
spectroscopy the 572 is designed with a great amount of
flexibility in order for the pulse to be interfaced with an
analyzer. A gated baseline restorer (BLR) circuit is
included in this output for improved performance at all
count rates. A switch on the front panel permits the
threshold for the restorer gate to be determined
automatically, according to the input noise level, or
manually, with a screwdriver adjustment. The switch also
has a center PZ Adj setting that can be used to eliminate
the BLR effect when making pole-zero adjustments.
The unipolar output dc level can be adjusted from -0.1 to
+0.1 V to set the zero intercept on the analyzer when direct
coupling is used. The bipolar output, with a 0 to 10 V
range, can be used for crossover timing or may be
preferred for spectroscopy when operating into ac-
coupled systems at high counting rates. Typical system
block diagrams for a variety of experiments are described
in Section 4.
Three general methods of termination are used. The
simplest of these is shunt termination at the receiving end
of the cable. A second method is series termination at the
sending end. The third is a combination of series and
shunt termination, where thecable impedance is matched
both in series at the sending end and in shunt at the
receiving end. The combination is most effective, but this
reduces the amount of signal strength at the receiving end
to 50% of that which is available in the sending instrument.
To use shunt termination at the receiving end of thecable,
connect the <1fl output of the 572 (on the front panel)
through 93fl cable to the input of the receiving instru
ment. Then use a BNC tee connector to attach both the
interconnecting cable and a 100(1 terminator at the input
connector of the receiving instrument. Since the input
impedance of the receiving instrument is normally 1000(1
or more, the effective instrument input impedance with
the 100(1 terminator will be of the order of 93(1 and this
matches the cable impedance correctly.
For series termination use the 93(1 output of the 572 for
the cable connection. Use 93(1 cable to interconnect this
into the input of the receiving instrument. The 1000(1 (or
more) normal input impedance at the input connector
represents an essentially open circuit, and the series
impedance in the 572 now provides the proper termina
tion for the cable.

For the combination of series and shunt termination, use
the 93il output on the rear panel of the 572 and use 931t
cable. At the input for the receiving instrument use a BNC
tee to attach both the signal cable and a lOOfl resistive
terminator. Note that the signal span at the receiving end
of this type of circuit will always be reduced to 50% of the
signal span furnished by the sending instrument.
For customer convenience, ORTEC stocks the proper
terminators and BNC tees, or they can be ordered from a
variety of commercial sources.
3.8. SHORTING OR OVERLOADING THE
AMPLIFIER OUTPUTS
All outputs of the 572 are dc-coupled with an output im
pedance of about 0.1 II for the front panel connectors and
9311 for the rear panel connectors. If the output is shorted
with a direct short circuit the output stage will limit the
peak current of the output so that the amplifier will not be
harmed. When the amplifier is terminated with 10011, the
maximum rate allowed to maintain the linear output is
[200 000 cps/r(MS)] X [10/V.,(V)].
3.9. INHIBIT OUTPUT CONNECTION
The Inhibit output on the rear panel is intended for
application at the anticoincidence input of the Analyzer.
An output pulse is generated through this connector
when a pulse pileup is sensed in the 572, and the pulse can
then be used to prevent the Analyzer from measuring and
storing a false amplitude. The signal is dc-coupled and
rises from 0 to about +5 V for a time equal to 6r, starting
when a pileup occurs.
3.10. BUSY OUTPUT CONNECTION
The signal through the rear panel Busy output connector
rises from 0 to about +5 V at the onset of each linear input
pulse. Its width is equal to the time the input pulse
amplitude exceeds the BLR discriminator level, and is
extended automatically by the generation of an Inhibit
output signal. It can be used to provide MCA live-time
correction, to control the generation of input pulses, to
observe normal operation with an oscilloscope, orforany
of a variety of other applications. Its use is optional and no
termination is required if the output is not being used.
3.11. CRM OUTPUT CONNECTION
One NIM-standard positive logic pulse is generated to
correspond to each linear input pulse into the 572. The
pulses are available through the CRM (Count Rate Meter)
output BNC on the rear panel and are intended for use in a
count rate meter or counter to monitor the true input
count rate into the amplifier. Its use is optional and no
termination is required if the output is not being used.
4. OPERATION
4.1. INITIAL TESTING AND OBSERVATION
OF PULSE WAVEFORMS
Refer to Section 6 for information on testing performance
and observing waveforms at front panel test points. Figure
4.1 shows some typical unipolar and bipolar output
waveforms.
4.2. FRONT PANEL CONTROLS
GAIN A coarse Gain switch and a Gain 10-turn locking
precision potentiometer select and precisely adjust the
gain factor forthe amplification in the 572. Switch settings
are X20, 50, 100, 200, 500, and 1000. Continuous fine gain
range is from X0.5 to XI .5, using markings of 500 through
1500 dial divisions. An internal jumper setting provides
one additional gain factor selection of either X1.0 or X0.1.
Collectively the range of gain can be set at any level from
XI.0 through X1500, using all three of these controls.
POS/NEG A toggle switch selects an input circuit that
accepts either polarity of pulses from the preamplifier.
PZ ADJ A screwdriver control to set the pole-zero
cancellation to match the preamplifier pulse decay
characteristics. The range is from 40 ijls to infinity.
DC A screwdriver control adjusts the dc baseline level of
the unipolar output in the range of -0.1 V to +0.1 V.
DELAY A toggle switch selects either prompt (Out) or 2
/iS delay timing for the unipolar output signals.
SHAPING A 6-position switch selects equal integrate
and differentiate time constants to shape the input pulses.
Settings are 0.5, 1, 2, 3, 6, and 10 /us.
BLR A 3-position toggle switch controls the operation of
the internal baseline restorer (BLR) circuit. The center
setting of the switch is effectively Off, and this permits
adjustment of the PZ Adj control without interference
from the BLR circuit. The Auto setting of the switch
selects a circuit that regulates the threshold of the BLR
gate according to the output noise level. The Threshold
setting permits manual control of the BLR gate threshold,
using the screwdriver control immediately below the
toggle switch.
4.3. FRONT PANEL CONNECTORS
INPUT Accepts input pulses to be shaped and/or
amplified by the 572. Compatible characteristics; positive
or negative with rise time from 10 to 650 ns; decay time

Shaping
Time 0.5 mS
Shaping
Time 1 us
Shaping
Fig. 4.1. Typical Effects of Shaping-Time Seiection
on Output Waveforms.
greater than 40 iis for proper pole-zero cancellation; input
linear amplitude range 0 to 10 V, with a maximum limit of
±20 V. Input impedance is approximately 500n.
UNI Provides a unipolar positive output with charac
teristics that are related to input peak amplitude, gain,
shaping time constants, pole-zero cancellation, and
baseline stabilization. The dc baseline level is adjustable
for offset to ±0.1 V. Timing can be either prompt or
delayed 2 with respect to the input. Output impedance
through this connector is about O.in, dc coupled. Linear
range 0 to ±10 V.
Bi Provides a bipolar pulse with positive lobe leading
and with characteristics that are related to input peak
amplitude, gain, and shaping time constants. Timing is
prompt with respect to the input. Crossover walk of this
output is less than 3 ns (see Specifications). Output
impedance through this connector is about O.lfl. Linear
range of 0 to ±10 V.
4.4. REAR PANEL CONNECTORS
IN Bridged with front panel Inputconnectorforoptional
use as an alternate input connection location.
INH Provides an output signal to identify a pulse pileup
in the 572. Connect it to the anticoincidence input of the
multichannel analyzer to inhibit measurement and
storage of false amplitudes.
UNI Provides a unipolar positive output with the same
characteristics as described for the front panel Uni
connector; the output impedance through this connector
is 93fl.
81 Provides a bipolar output with the same charac
teristics as described for the front panel Bi connector;
the output impedance through this connector is 93n.
CRM Provides a NIM-standard (s+5 V) slow positive
logic output for each linear input pulse. The output can be
connected into a ratemeter or counter to monitor the true
input count rate for the amplifier.
BUSY Provides a signal that rises to approximately ±5 V
for the time that the input pulse amplitude exceeds the
BLR discriminator level, which can be controlled
manually or automatically. The output can be used to
correct for live time in the ORTEC MCA by connecting it
to the MCA Busy input.
PREAMP Provides power connections from the bin and
power supply to the ORTEC preamplifier. The dc levels
include ±24 V, -24 V, ±12 V, and -12 V.
4.5. STANDARD SETUP PROCEDURE
a. Connect the detector, preamplifier, high voltage
power supply, and amplifier into a basic system and con-
net the amplifier unipolar output to an oscilloscope. Con
nect the preamplifier power cable to the Preamp power
connector on the rear panel of the 572. Turn on power in

the Bin and Power Supply and allow the electronics of the
system to warm up and stabilize.
b. Set the 572 controls initially as follows:
Shaping
Coarse Gain
Gain
Internal Jumper
BLR
Thresh
Pos/Neg
2 ns
50
1.000
X1.0
PZ Adj
Fully clockwise
Match preamplifier
output polarity
c. Use a ^°Go calibration source, set about 25 cm from
the active face of the detector. The unipolar output pulse
from the 572 should be about 8 to 10 V, using a pream
plifier with a conversion gain of 170 mV/MeV.
d. Readjust the Gain control so that the higher peak
from the ^"Co source (1.33 MeV) provides an amplifier
output at about 9 V.
4.6. POLE-ZERO ADJUSTMENT
The pole-zero adjustment is extremely critical for good
performance at high count rates. This adjustment should
be checked carefully for the best possible results.
USING Ge(Li) SYSTEM AND "Co
a. Adjust the radiation source count rate between 2 kHz
and 10 kHz.
b. Observe the unipolar output with an oscilloscope.
Adjust the PZ Adj control so that the trailing edge of the
pulses returns to the baseline without overshoot or
undershoot (see Fig. 4.2).
The oscilloscope used must be dc-coupled and must not
contribute distortion in the observed waveforms.
Oscilloscopes such as Tektronix 453, 454, 465, and 475
will overload for a 10-V signal when the vertical
sensitivity is less than 100 mV/cm. To prevent overload
ing the oscilloscope, use the clamp circuit shown in Fig.
4.3.
USING SQUARE WAVE THROUGH PREAMPLIFIER
TEST INPUT
A more precise pole-zero adjustment in the 572 can be
obtained by using a square wave signal as the input to
the preamplifier. Many oscilloscopes include a calibra
tion output on the front panel and this is a good source of
square wave signals at a frequency of about 1 kHz. The
amplifier differentiates the signal from the preamplifier
so that it generates output signals of alternate polarities
on the leading and trailing edges of the square wave
input signal, and these can be compared as shown in Fig.
4.4 to achieve excellent pole-zero cancellation.
Overcompensated
Adjusted
Properly
Undercompensated
Fig. 4.2. Typical Waveforms Illustrating Pole-Zero Adjustment
Effects; Oscilloscope Trigger, 572 Busy Output; ""Co
Source with 1.33-MeV Peak Adjusted ~9 V;
Count Rate, 3 kHz; Shaping Time Constant, 2

10
FROM AMPLIFIER
OUTPUT IK
TO OSCILLOSCOPE
INPUT
AAA^
D
r
HPA 2800
SCHOTTKY DIODES
OR FAST Ge DIODES
Fig. 4.3. A Clamp Circuit ttiat Can Be Used to Prevent
Overloading ttie Oscilloscope Input.
Use the following procedure:
a. Remove all radioactive sources from the vicinity of
the detector. Set up the system as for normal operation,
including detector bias.
b. Set the 572 controis as for normal operation; this
includes gain, shaping, and input polarity.
c. Connect the source of 1-kHz square waves through
an attenuator to the Test input of the preamplifier. Adjust
the attenuator so that the 572 output amplitude is about
9 V.
d. Observe the Unipolar output of the 572 with an
oscilloscope, triggered from the 572 Busy output. Adjust
the PZ Adj control for proper response according to Fig.
4.4. Use the clamp circuit of Fig. 4.3 to prevent over
loading the oscilloscope input.
Figure 4.4A shows the amplifier output as a series of
alternate positive and negative Gaussian puises. In the
other three pictures of this figure the oscilloscope was
triggered to show both positive and negative pulses
simultaneously. These pictures show more detail to aid
in proper adjustment.
av.
A. PZ Properly Adjusted; Slow Trigger to Separate Pulses.
B. Overcompensated; Fast Trigger to Superimpose Pulses.
C. Properly Adjusted; Pulses Superimposed. D. Undercompensated; Pulses Superimposed.
Fig. 4.4. Pole-Zero Adjustment Using a Square Wave Input to the Preamplifier.

11
4.7. BLR THRESHOLD ADJUSTMENT
After the amplifier gain and shaping have been seiected
and the PZ Adj control has been set to operate properly
for the particular shaping time, the BLR Thresh control
can be used to establish the correct discriminator
threshold for the baseline restorer circuit. Normally, the
toggle switch can be set at Auto and the threshold level
will be set automatically just above the noise level. If
desired, the switch can be set at Threshold and the
manual control just below the switch can then be used to
select the level manually as follows:
a. Remove all radioactive sources from the vicinity of
the detector. Set up the system as for normal operation,
including detector bias.
b. Set the BLR switch at Threshold and turn the control
fully clockwise, for 300 mV.
c. Observe the unipolar output on the 100 mV/cm scale
of the oscilloscope, using 5 /us/cm horizontal deflection.
Trigger the oscilloscope with the Busy output from the
572.
d. Reduce the control setting until the baseline
discriminator begins to trigger on noise; this corresponds
to about 200 counts/s from the Busy output. Adjust the
trigger ievel according to the information in Fig. 4.5.
If a ratemeter or counter-timer is available, it can be
connected to the Busy output and the threshold level can
then be adjusted for about 200 counts/s.
4.8. OPERATION WITH SEMICONDUCTOR
DETECTORS
CALIBRATION OF TEST PULSER An ORTEC 419 Pre
cision Pulse Generator, or equivalent, is easily calibrated
so that the maximum pulse height dial reading (1000
divisions) is equivalent to a 10-MeV loss in a silicon
radiation detector. The procedure is as foilows:
a. Connect the detector to be used to the spectrometer
system, i.e., preamplifier, main amplifier, and biased
ampiifier.
b. Allow excitation from a source of known energy (for
example, alpha particles) to fall on the detector.
c. Adjust the amplifier gain and the bias level of the
biased amplifier to give a suitable output pulse.
d. Set the pulser Pulse Height control at the energy of
the alpha particles striking the detector (for example, set
the dial at 547 divisions for a 5.47-MeV alpha particle
energy).
e. Turn on the pulser and use its Normalize control and
attenuators to set the output due to the pulser for the same
pulse height as the pulse obtained in step c. Lock the
Normalize control and do not move it again untii recalibra-
tion is required.
The pulser is now calibrated; the Pulse Height dial reads
directly in MeV if the number of dial divisions is divided by
100.
A. Set too Higl
B. Adjusted Properly
C. Set too Low
Fig. 4.5. BLR Tlireshold Variable Control Settings.

12
AMPLIFIER NOISE AND RESOLUTION MEASURE
MENTS As shown in Fig. 4.6, a preamplifier, amplifier,
pulse generator, oscilloscope, and wide-band rms volt
meter such as the Hewlett-Packard 3400A are required
for this measurement. Connect a suitable capacitor to
the input to simulate the detector capacitance desired.
To obtain the resolution spread due to amplifier noise:
a. Measure the rms noise voltage (Ems) at the amplifier
output.
b. Turn on the 419 Precision Pulse Generator and
adjust the pulser output to any convenient readable
voltage, E„, as determined by the oscilloscope.
The full width at half maximum (FWHM) resolution
spread due to amplifier noise is then
2.35 Erms Edtal
/V(FWHM) = ,
E„
where Ed,ai is the pulser dial reading in MeV and 2.35 is
the factor for rms to FWHM. For average-responding
voltmeters such as the Hewlett-Packard 400D, the
measured noise must be multiplied by 1.13 to calculate
the rms noise.
ORTEC ORTEC
LINEAR
AMPLIFIER
OSCILLO
SCOPE
PREAMP
. DETECTOR
. OR
CAPACITOR
PULSE
GENERATOR
VOLTMETER
Fig. 4.6. System for Measuring Amplifier and Detector
Noise Resolution.
The resolution spread will depend on the total input
capacitance, since the capacitance degrades the signal-
to-noise ratio much faster than the noise.
DETECTOR NOISE-RESOLUTION MEASUREMENTS
The measurement just described can be made with a
biased detector instead of the external capacitor that
would be used to simulate detector capacitance. The
resolution spread will be larger because the detector
contributes both noise and capacitance to the input. The
detector noise-resolution spread can be isolated from
the amplifier noise spread if the detector capacity is
known, since
(Nd..)' + (A/amp)' = (W.o.al)' ,
where Moiai is the total resolution spread and A/amp is the
amplifier resolution spread when the detector is replaced
by its equivalent capacitance.
The detector noise tends to increase with bias voltage,
but the detector capacitance decreases, thus reducing
the resolution spread. The overall resolution spread will
depend upon which effect is dominant. Figure 4.7 shows
curves of typical noise-resolution spread versus bias
voltage, using data from several ORTEG silicon surface-
barrier semiconductor radiation detectors.
o 0.4
ORTEG BA-030-007-300
ORTEG BA-025-050-100
ORTEG BA-025-100-1C0
ORTEG BA-030-200-100
ORTEG BA-045-450-100
50
Bias Voltage
Fig. 4.7. Noise as a Function of Bias Voltage.
AMPLIFIER NOISE-RESOLUTION MEASUREMENTS
USING MCA Probably the most convenient method of
making resolution measurements is with a pulse height
analyzer as shown by the setup illustrated in Fig. 4.8.
The amplifier noise-resolution spread can be measured
directly with a pulse height analyzer and the mercury
pulser as follows:
a. Select the energy of interest with an ORTEG 419
Precision Pulse Generator. Set the amplifier and biased
amplifier gain and bias level controls so that the energy
is in a convenient channel of the analyzer.
100
DETECTOR
OR
CAPACITOR
r——1
I ORTEC 1
I GATED BIAS I
> AMPLIFIER j
L. , J
PULSE
GENERATOR
ORTEC
ORTEC
MULTI
CHANNEL
PULSE HEIGHT
ANALYZER
PREAMP
ORTEC
LINEAR
AMPLIFIER
ORTEC
BIASED
AMPLIFIER
ORTEC
PULSE
STRETCHER
ORTEC
Fig. 4.8. System for Measuring Resolution with a
Pulse Height Analyzer.

b. Calibrate the analyzer in keV per channel, using the
pulser; full scale on the pulser dial is 10 MeV when
calibrated as described above.
c. Obtain the amplifier noise-resolution spread by
measuring the FWHM of the pulser peak in the spectrum.
The detector noise-resolution spread for a given detector
bias can be determined in the same manner by connect
ing a detector to the preamplifier input. The amplifier
noise-resolution spread must be subtracted as described
in "Detector Noise-Resolution Measurements." The
detector noise will vary with detector size and bias
conditions and possibly with ambient conditions.
CURRENT-VOLTAGE MEASUREMENTS FOR SI and Ge
DETECTORS The amplifier system is not directly
involved in semiconductor detector current-voltage
measurements, but the amplifier serves to permit noise
monitoring during the setup. The detector noise measure
ment is a more sensitive method than a current measure
ment of determining the maximum detector voltage that
should be used because the noise increases more rapidly
than the reverse current at the onset of detector
breakdown. Make this measurement in the absence of a
source.
Figure 4.9 shows the setup required for current-voltage
measurements. An ORTEC 428 Bias Supply is used as the
voltage source. Bias voltage should be applied slowly and
reduced when noise increases rapidly as a function of
applied bias. Figure 4.10 shows several typical current-
voltage curves for ORTEC silicon surface-barrier detec
tors.
When it is possible to float the microammeter at the
detector bias voltage, the method of detector current
measurement shown by the dashed lines in Fig. 4.9 is
preferable. The detector is grounded as in normal
operation and the microammeter is connected to the
current monitoring jack on the 428 Detector Bias Supply.
DETECTOR
BIAS SUPPLY
1
I
j
\ 1
AMMETER j
I
Current Monitoring Jacks
ORTEC
PREAMP
ORTEC
LINEAR
AMPLIFIER
RMS
VOLTMETER
DETECTOR
Normal
Circuit
MICRO
AMMETER
BYPASS
- CAPACITOR
Fig. 4.9. System for Detector Current and Voltage
Measurements.
0.4
0.2
4040
60
60
80
100
A - ORTEC BA-030 007-300
B - ORTEC BA-025 050-100
C - ORTEC BA 025-100-100
D - ORTEC BA-030.200-100
E - ORTEC BA-045-450-100
Bias Voltage
Fig. 4.10. Silicon Detector Back Current vs Bias Voltage.
4.9. OPERATION IN SPECTROSCOPY SYSTEMS
HIGH-RESOLUTION ALPHA-PARTICLE SPECTROS
COPY SYSTEM The block diagram of a high-resolution
spectroscopy system for measuring natural alpha particle
radiation is shown in Fig. 4.11. Since natural alpha radia
tion occurs only above several MeV, an ORTEC 444 Bi
ased Amplifier is used to suppress the unused portion of
the spectrum; the same result can be obtained by using
digital suppression on the MCA in many cases. Alpha-
particle resolution is obtained in the following manner:
a. Use appropriate amplifier gain and minimum biased
amplifier gain and bias level. Accumulate the alpha peak
in the MCA.
b. Slowly increase the bias level and biased amplifier
gain until the alpha peak is spread over 5 to 10 channels
and the minimum- to maximum-energy range desired
corresponds to the first and last channels of the MCA.
c. Calibrate the analyzer in keV per channel using the
pulser and the known energy of the alpha peak (see
"Calibration of Test Pulser") or two known energy alpha
peaks.
d. Calculate the resolution by measuring the number of
channels at the FWHM level in the peak and converting
this to keV.
HIGH-RESOLUTION GAMMA SPECTROSCOPY SYS
TEM A high-resolution gamma spectroscopy system
block diagram is shown in Fig. 4.12. Although a biased
amplifier is not shown (an analyzer with more channels
being preferred), it can be used if the only analyzer
available has fewer channels and only higher energies are
of interest.
When germanium detectors that are cooled by a liquid
nitrogen cryostat are used, it is possible to obtain
resolutions from about 1 keV FWHM up (depending on the
energy of the incident radiation and the size and quality of
the detector). Reasonable care is required to obtain such
results. Some guidelines for obtaining optimum resolu
tion are:
a. Keep interconnection capacities between the detec
tor and preamplifier to an absolute minimum (no long
cables). ^