ORTEC 451 User manual

INSTRUCTION MANUAL
451
SPECTROSCOPY AMPLIFIER
Serial No.
Purchaser
Date Issued
100 MIOLANO ROAD
OAK RIDGE. TENN. 37830
PHONE I6151 482-4411
TWX 810-572-1078

TABLE OF CONTENTS
Page
WARRANTY
PHOTOGRAPH
1. DESCRIPTION
1.1 General Description
1.2 PolsZero Cancellation
1.3 Active Filter
l-l
I-1
l-2
1-2
2. SPECIFICATIONS
2.1 Electrical
2-1
2-1
3. INSTALLATION 3-1
3.1 General Installation Considerations
3.2 Connection to Preamplifier
3.3 Connection of Test Pulse Generator
3.4 Connection to Power - Nuclear Standard Bin, ORTEC 401A/402A
3.5 Shaping Considerations
3.6 Use of Delayed Output
3.1 Output Connections and Tern+nating Considerations
3.8 Shorting 01 Overloading the Amplifier Outputs
4. OPERATING INSTRUCTIONS
3.1
3-1
3-1
3-2
3-2
3-2
3-2
3-3
4-l
4.1 Front Panel Controls 4-1
4.2 Rear Panel Controls 4.1
4.3 Internal Contmlr 4-1
4.4 Fmnt Panel Connectors (All Type BNCI 4.1
4.5 Rear Panel Connectors 4-3
4.6 Initial Testing and Observation of Pulse Waveforms 4-3
4.7 General Considerations for Operation with Semiconductor Detectors 4-3
4.8 Operation in Spectroscopy Systems 4.10
4.8 Typical System Block Diagrams (Figurer 4-12 through 415) 4.11
4.10 Baseline Restorer (BLR) 4-11
4.11 Methods of Connection to Various Analyzers 4.18
6. CIRCUIT DESCRIPTION 5-1
6. MAINTENANCE 6-1
8.1 Test Equipment Required 6-1
8.2 Pulser Modifications for Overload Tests 6-1
6.3 Pulser Testr 6-1
6.4 Suggestions for Troubleshooting 6-3
8.5 Tabulated Test Point Voltages on Etched Board 8-4

LISTOF FIGURESAND ILLUSTRATIONS
Figure l-1 Clipping in a Non-PoleZero Can&led Amplifier 1-3
Figure l-2 Differentiation (Clipping) in a PoleZero Cancelled Amplifier 1-3
Figure 1-3 Pulse Shapes for Good Signal-to-Noise Ratios 1-4
Figure 4-l Effects of Shaping Time Selection on Output Waveforms 4-2
Figure 42 Measuring Amplifier end Detector Noise Resolution 4-4
Figure 4-3 Resolution Effects of Capacitance 4-6
Figure 4-4 Noise as a Function of Bias Voltage 4-6
Figure 4.5 System For Measuring Resolution With a Pulse Height Analyzer 4-7
Figure 46 System For Detector Current and Voltage Measurements 4-8
Figure 47 Silicon Detector Back Current Versus Bias Voltage 4-8
Figure 4-8 System For High Resolution Alpha Particle Spectmscopy 4-9
Figure 49 System For High Resolution Gamma Spectroscopy 4-9
Figure 410 Scintillation Counter Gamma Spectroscopy System 4.12
Figure411 High Resolution X-Ray Spectroscopy System 4.12
Figure 4-12 Gamma-Gamma Coincidema Experiment-Block Diagram 4-13
Figure 413 Gamma Ray-Charged Particle Coincidence Experiment-Block Diagrams 4-14
Figure 4-14 Gamma Ray Pair Spectrometer-Block Diagrams 4.14
Figure4.15 General System Arrangement for Gating Control 4.15
Figure 4-16 Analyzer Connection With No Trigger Required 4-16
Figure 417 Analyzer Connection When Trigger is Required 4.16
Figure 418 Effects of Baseline Restorer on Resolution 4.17

A NEW STANDARD TWO-YEAR WARRANTY FOR ORTEC ELECTRONIC INSTRUMENTS
ORTEC warrants its nuclear instrument products to be free from defects in workmanship and materials, other
than vacuum tubss and semiconductors, for a period of twenty-four months from date of shipment, provided
that the equipment has been used in a proper manner and not subjected to abuse. Repairs or replacement, et
ORTEC optil~ n, will be made without charge at the ORTEC factory. Shipping expense will be to the account
of the customer except in cases of defects discovered upon initial operation. Warranties of vacuum tubes and
semiconductors, as made by their manufacturers, will be extended to our customers only to the extent of the
manufacturers’ liability to OATEC. Specially selected vacuum tubes or semiconductors cannot be warranted.
ORTEC reservestheright to modify the design of its products without incurring responsibility for modification
of previously manufactured units. Since installation conditions are beyond our control. ORTEC does not
assume any risks or liabilities associated with methods of installation other than specified in the instructions,
or imtallation results.
OUALITY CONTROL
Before being approved for shipment, each ORTEC instrument must pass a stringent set of quality control tests
designed to expose any flaws in materials or workmanship. Pwmanent records of these tests are maintained for
use in warranty repair and as a source of statistical information for design improvements.
REPAIR SERVICE
ORTEC instruments not in warranty may be returned to the factory for repairs or checkout at modest expense
to the customer. Standard procedure requires that returned instruments pass the same quality control tets as
those used for new production instruments. Please contact the factory for instructions before shipping
equipment.
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 ORTEC of the circumstances so
that we may assist in damage claims and in providing replacement equipment if necessary.

!)?I u !L3 ll
IIE!;TOIIE
IEUY HI

ORTEC 451
SPECTROSCOPY AMPLIFIER
1-1
1. DESCRIPTION
1.1 General Description
The ORTEC 451 Spectroscopy Amplifier is a single width NIM module with a versatile combination
of switch selectable pulse shaping and output 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 which originate in germanium or silicon semiconductor detectors.
scintillation detectors with either fast or slow scintillaton, proportional counters. pulsed ionization
chambers, electron multiplixs, etc.
The 451 has a dc input impedance for approximately 1000 ohms and accepts either positive or negative
input pulses with rise times < 650 nsec and fall times > 25!~sec. Three integrate and differentiate time
constants are separately switch selectable to provide optimum shape for resolution and count rate. The
first differentiation network has variable pole-zero cancellation which can be adjusted to match
preamplifiers with > 25psec decay time. The pole-zero cancellation drastically reduces the undershoot
after the first clip and greatly improves overload characteristics. In addition, the amplifier contains an
active filter shaping network which 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 451
Amplifier to the analyzer. A BLR (Base Line Restoration) circuit is included in the 451 for improved
performance at high count rates. A switch on the rear panel permits this circuit to be switched out. set
for low count rates, or set for high count rates. When using the direct coupled input of the various
analyzers, a variety of voltage requirements exist. To meet these requirements the 451 unipolar out!
put can be selected for either positive or negative polarity and selected for full scale voltage of 3V,
6V. or 1OV. The unipolar output dc level can be adjusted from -IV to +lV. This output permits the
use of the direct coupled input of analyzers with a minimum amount of interface problems. The 451
bipolar output may be preferable for spectroscopy when operating into an x-coupled system at high
counting rates.
The 451 can be used for crossover timing when used in conjunction with an ORTEC 407 Crossover Pick-
off or a 420A Timing Single Channel Analyzer. The 420A Timing Single Channel Analyzer output has a
minimum of walk as a function of pulse amplitude and incorporates a variable delay time on the output
pulse to enable the crosswer pickoff output to be placed in time coincidence with other outputs. A
switch selectable 2psec delay is provided on the unipolar output to aid in obtaining the proper spacing
of the linear pulse in a coincidence gated system.
The 451 has complete provisions including power for operating any ORTEC solid state preamplifier
such as the 109A. 113, 118A. and 120. Preamplifier pulses should have a rise time of 0.5jJsec or less, to
properly match the amplifier filter network and a decay time > 25!.1sec for proper pole-zero cancella-
tion. The 451 input impedance is 1000 ohms. 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 of the 451 is about 0.1 ohm at the front panel connectors and 93 ohms at the
rear panel connectors. The front panel outputs can be connected to other equipment by single cable
going to all equipment and shunt terminated at the far end. If series termination is desired, the rear
panel connectors can be used in connecting the 451 to other modules (see Section 3).
Gain changing is accomplished by varying feedback networks. These networks are varied in such a
manner that the band width of the fedback amplifier stages remain essentially constant regardless to
gain and, therefore, rise time changes with gain switching (which cause crossover walk) are limited to
small variations.

1-2
1.2 PoleZero Cancellation
Pole-zero cancellation is a method for eliminating pulse undershoot after the first differentiating “et-
work. The technique employed is dexribed by referring to the waveforms and equations show” in
Figures l-l and 1-2. In a nonpole-zero cancelled amplifier, the exponential tail on the preamplifier out-
put signal (usually 50 to 500&1sec) causes a” undershoot whose peak amplitude is roughly:
Undershoot Amplitude Differentiation Time
Differentiated Pulse Amplitude Preamplifier Pulse Decay Time
For a lpsec differentiation time and a 5Opsec preamplifier pulse decay time. the mazimum undershoot
is 2% and decays with a 50!.1sec time constant. Under overload conditions, this undershoot is often
sufficiently large to saturate the amplifier during a considerable portion of the undershoot causing
excessive deadtime. This effect can be reduced by increasing the preamplifier pulse decay time (which
generally redu&s 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 Figure 1-2.
The pole ho) due to the preamplifier pulse decay time is cancelled by the zero (S + K&C, I of
the network. In effect, the dc path across the differentiation capacitor adds a” attenuated replica of the
preamplifier pulse to just cancel the negative undershoot of the clipping network.
Total preamplifier - amplifier pole-zero cancellation requires that the preamplifier output pulsedecay
time is a single exponential decay and matched to the pole-zero cancellation network. The variable pole-
ZWCI c~ncellatio” network allows accurate cancellation for all preamplifiers having 25~~ or greater
decay times. The network is factory adjusted to 50~s~ which is compatible with all ORTEC FET
preamplifiers. Improper matching of the pole-zero cancellation network will degrade the overload
performance and cause excaive pile-up distortion at medium counting rates. Improper matching
causes either a” undercompensation (undershoot is not eliminated) or a” overcompensation (output
after the main pulse does not return to the baseline and decays to the baseline with the preamplifier
time constant). The pole-zero adjust is accessible from the front panel of the 451 and can easily be
adjusted by observing the baseline with a monoenergetic source or pulser having the same decay time
as the preamplifier under overload conditions.
1.3 Active Filter
When only FET gate current and drain thermal noise are considered, the best signal-to-noise ratio occurs
where the two noise contributions are equal for a give” pulse shape. The Gaussian pulse shape of this
amplifier requires a single RC differentiate and n equal RC integrates where n approaches infinity. The
Laplace transform of this transfer function is:
S 1
G’S) = (S + l/RC) x (S + l/RC)” “---
where the first factor is the single differentiate and the second factor is the n integrates. The 451 Active
filter approximates this transfer function.

1-3
F
Prsamplifier X Fir*, Clipped
0”tp”t Amplifier P”lW
Clipping = with
Netwok Undershoot
Equationr:
E?e -t/To X G(t) = e, 0)
(Ea) (+) X ( s + ,,& ) = e, (5) i W=e Tmnsfmm
&. [T,e-t’T1-Tl.-“T,l = e,(t); 1, = R,C,
Figure l-l. Clipping in a Non-PoleZem Cancellad Amplifier
TT&- = s+R;G = h(l); whRp’ **
Fiwm l-2. Diffemntiation (clipping) in a P&Zen, Cancrlld Amptifin

1-4
CUSP -t/lx
e ,t>0
t/RC
e ,t<ll
GAUSSIAN s 1
(s+l/Rc) 6+1/k” “--==
ACTIVEFILTER (S+& ($-i;++~)
j=fi
Figure 1-3 P+a Shapes for Good Sinai-tddoira Ratios