EG&G ORTEC Gamma-X Service manual

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Gamma-X®
High-Purity Germanium (HPGe)
Coaxial Detector Systems
Preliminary Operating and
Service Manual
Operating and Service Manual
PRELIMINARY
®Trademark owned by EG&G ORTEC.

ii Gamma-X
IMPORTANT INFORMATION
1. Read carefully the standard instructions for unpacking the
system and filling it with liquid nitrogen, and the checkout
procedures that are included in Sections 2 and 3 of this manual.
2. Fill the dewar with liquid nitrogen and allow at least six
hours for cooling before connecting the system to any external
electronics. ALWAYS disconnect the system from external electronics
before allowing it to warm up. NEVER connect the system to external
electronics before the detector has been cooled properly. If high
voltage is applied to the detector at any time when the detector is
not at liquid nitrogen temperature, the input FET of the preamplifier
may be damaged, voiding the warranty. A damaged input FET will be
considered "prima facie" evidence that high voltage has been applied
when the system was not properly cooled.
3. NEVER accelerate a warmup of the cryostat by heating it with
an external source.
4. The plastic bag of desiccant pellets over the cryostat-
preamplifier assembly absorbs moisture that condenses when the
system is moved from a cold area into a warm area. Leave this bag
in place until the system has been in the warmer atmosphere long
enough to eliminate any danger of condensation.
5. The cylindrical protrusion (3/4 in. diam by 1 in. long)
that is on the cryostat is the vacuum pump attachment. Do not
tamper with this at any time.
6. Unlike other types of coaxial germanium detectors, the
EG&G ORTEC Gamma-X detector requires negative bias.
7. NEVER warm to room temperature a Gamma-X detector that shows
signs of radiation damage (see Sections 4 and 5 of this manual).
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Gamma-X
111
QUALITY ASSURANCE DATA
Gairana-X High-Purity Ge Coaxial Detector System
GENERAL DATA
Model No.6A)X-3oaiO-5 Serial No. 3l|-M-74^B
Shipping Date ^-3 i
PHYSICAL MEASUREMENTS
mm { Absorbing Layers:
mm j Aluminum ^ ^
cc ISO Ge Dead Layer D. "hM-
Window to Detector 5.0mm Teflon O mm
Diameter
Length
Active Vol
mm
Total Capacitance —'
ELECTRICAL DATA
pF at V.
Recommended Operating Bias, NEGATIVE j" ooo V.
Preamplifier Input FET (Cooled) (Not coolod).
PERFORMANCE SPECIFICATIONS
Warranted Measured
Efficiency at 1.33 MeV
(1)
30
%
Resolution at 1.33 MeV
.(1)
(
Resolution at 88 keV
1)
Resolution at 122 keV
(
_keV
eV
1)
^keV
Ratio of 22 keV counts to
109
88 keV counts from Cd
31.3 %
I 6 keV
eV
170 eV
OH
Amplifier
Time Constant
(fi PS
(o us
us
(p us
uS
(1) Measured in accordance with IEEE Standard 645-1977 (Supplement to
IEEE Standard 645-1977 (Supplement to IEEE Std 325-1971, Reaff 1977)
using EG&G ORTEC standard electronics.
Data Certified by HJa
Date
CAUTION
Models 20 and 21 cryostats can be damaged by unnecessary
overfilling if it is continued long enough to lower the
temperature of the flange appreciably. Never allow an
automatic filling to proceed without supervision. Any
extended overfilling of the LN2 can freeze the Viton 0-
ring and cause loss of vacuum, which may seriously damage
the detector.
Gamma-X is a Trademark owned by EG&G ORTEC

iv Gairana-X
GAMMA-X HIGH-PURITY GERMANIUM (HPGe) COAXIAL DETECTOR SYSTEMS
Warranty Statement
EG&G ORTEC warrants its Gamma-X HPGe Coaxial Detector Systems to
be free from defects in material and workmanship for a period of one
year after shipment. EG&G ORTEC guarantees that the detector system
will operate within the warranted specifications regardless of the
number of thermal cycles between liquid nitrogen temperature and room
temperature to which the system has been subjected and regardless of
how long the system has been stored at room temperature during the
one-year period. This warranty is subject to the following customer
obligations:
1. The cryogenic FET has not been damaged.
2. The system has not been physically or electrically abused.
3. The detector has not been subjected to neutron damage.
Should the detector system fail, through no fault of the customer,
during the warranty period, and having at all times been handled in
accordance with the above described customer obligations, it will be
repaired or replaced without charge, at EG&G ORTEC's option, and will
be fully warranted to within the original specifications for three
months or for the remainder of the warranty period, whichever is longer.
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 agreement 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 no event shall EG&G ORTEC be liable for special or consequential
damages. EG&G ORTEC makes no other warranties, expressed or implied,
and specifically NO WARRANTY OF MERCHANTABILITY OR FITNESS FOR A
PARTICULAR PURPOSE.

Gamma-X v
Instructions for Returning a Gamma-X HPGe Detector Coaxial Detector System
If, for any reason, it becomes necessary to return a Gamma-X Coaxial
Detector System for repair or replacement, please note the following
Instructions and precautions regarding shipment.
!• Contact EG&G ORTEC Contact the nearest EG&G ORTEC representative
[in the U.S. call Customer Services, EG&G ORTEC Physical Sciences
Division, [phone]] for specific Instructions and
authorization for returning the detector. Any detector that
Is returned without prior notification having been given to
EG&G ORTEC could result In loss or damage and would be
automatically considered out-of-warranty.
2. Proper Packing A detector "returns kit" must be utilized for
return shipment. The kit contains the necessary Instructions
and material to ensure proper packing. Contact the nearest EG&G
ORTEC representative for the correct returns kit for your particular
detector before attempting return. Improper packing could result
In shipment damage that would not be covered under the EG&G ORTEC
warranty or the freight forwarder's Insurance policy.
3. Shipment Coordination and Communications Detectors should be
returned through the freight forwarder collect. For out-of-warranty
repairs, the shipping charges will be billed to the customer.
For In-warranty repairs, the reshlpment charges will be paid by
the customer. Weekend delays In transit should be avoided; thus
an early weekday shipment Is recommended. We highly recommend
that the customer utilize the EG&G ORTEC preferred forwarder who
Is familiar with shipment of HPGe Coaxial detectors. EG&G ORTEC
must be notified of specific details of the returning shipment
(I.e. , freight forwarder, air freight carrier, date, time, airway
bill number, etc.) at the time of shipment to ensure attention
upon receipt at the repair facility.
The Instructions and precautions stated herein are to ensure proper
attention In handling the shipment of an expensive radiation detector. Do
not be apprehensive because of these precautions. Detectors are shipped
successfully all over the world when just these few precautions are followed.
When In doubt about any of the above-mentioned precautions or Instructions,
please contact the nearest EG&G ORTEC representative for assistance.

vi Gamma—X
TABLE OF CONTENTS
1. GENERAL 1
1.1. The Gamma-X Detector Concept 1
1.2. Cryostat Configuration 2
1.3. Accidental Warm Up Protection 4
1.4. Spectroscopy with Gamma-X Detectors 5
2. RECEIVING, UNPACKING, AND FILLING WITH LIQUID NITROGEN 7
2.1. General 7
2.2. Unpacking and Filling with Liquid Nitrogen 7
3. CHECKOUT AND OPERATION 11
3.1. General 11
3.2. Test Before Application of Bias 13
3.3. Detector Resolution Measurements 14
3.4. Peak-to-Compton Measurements 16
3.5. Efficiency Measurement 17
3.6. ^'^^Cd Source Measurements 18
4. NEUTRON RADIATION DAMAGE 19
4.1. General 19
4.2. Neutron Radiation Damage in the Gamma-X Detector 19
5. MAINTENANCE AND TROUBLESHOOTING 21
5.1. Liquid Nitrogen 21
5.2. Packing for Reshipment 22
5.3. Troubleshooting Suggestions 22
APPENDIX - REFERENCES 29

Ganma-X
1. GENERAL
1.1. THE GAMMA-X^ DETECTOR CONCEPT
The EG&G ORTEC Gainina-X Detector is based on a new concept in
coaxial germanium detector geometries.^ Therefore a brief description
of this concept will facilitate the use of this instruction manual.
Four different configurations of germanium detectors, both Ge(Li)
and HPGe, are shown in Fig. 1.1. It is immediately apparent that the
Gamma-X detector structure is the only one in which the outer contact
is sufficiently thin to allow low energy x-rays to penetrate into the
active volume of the detector. For practical reasons, this type of
structure is presently the best implemented by using N- rather than
P-type HPGe material.
500-1000 MICRONS
300 600 MICRONS
RADIATION
RADIATION
P-TYPE CORE
CONVENTIONAL COAXIAL HP G«
CLOSED END G*(Li)
0.3 MICRONS
N-TYPE
HP Ge
100-200 MICRONS
RADIATION
RADIATION
P-TYPE CORE
P-TYPE
HP G«
TRUE COAXIAL G«<Li)
Very Thick Contact -300-1000 MIcront
Thick Contact -100- 200 Microns
Very Thin Contact 0,3 Microns
Fig. 1.1. Four Different Configurations of Coaxial Germanium Detectors
In the Gamma-X structure, a "thick" strongly N-type (Li-diffused)
contact is used as the inner electrode while a thin, ion implanted,
strongly P-type contact is used as the outer electrode.
Bias voltage for all the coaxial detectors shown in Fig. 1.1 is
applied to the outer electrode, while the signal is picked up from the
inner electrode. Since the positions of the P-type and N-type materials

2 Gamma-X
are reversed in the Ganma-X detector, this type of detector requires
a negative bias voltage. Consequently, the polarity of the signal from
the inner electrode is always negative, rather than positive as in
other types of coaxial germanium detectors.
Except for the negative bias voltage polarity and the negative charge
signal polarity, the operation of a Garama-X detector is identical to
the operation of any other germanium coaxial detector. More details
on the Gamma-X detector can be found in reference 1 and in EG&G ORTEC
catalogs and Product News literature.
Another outstanding feature of the Gamma-X detector is its high
degree of resistance to neutron radiation damage. This is discussed
in Section 4.
1.2. CRYOSTAT CONFIGURATION
A standard EG&G ORTEC Gamma-X Coaxial Detector system may have any
of the basic configurations shown in Fig. 1.2. The main difference
between these systems is the location of the active detector element.
In each configuration the HPGe coaxial detector element is located
concentrically in the end cap with its front face approximately 5 mm
from the outer surface of the end.cap.
The preamplifier and the high-voltage filter are connected electrically
to the detector through vacuum seals. The cryostat is designed to keep
the detector at liquid nitrogen temperature and in a sealed vacuum for
many years, if normal precautions are taken. In most HPGe Coaxial
Systems the first FET of the preamplifier is mounted inside the end cap
and cooled to achieve the best possible resolution. If this be the case,
it is so indicated on the Quality Assurance Data Sheet on page iii of
this manual.

Gamma-X
ff t* ORTEC GAMMA"*
'.... -A.
^4gBgf3 ORTEC GAMMA;£J
4\ Ti I iiiH
Fig. 1.2. Physical Appearance of Typical Gamma-X Detectors

4 Gamma-X
1.3. ACCIDENTAL WARM UP PROTECTION
EG&G ORTEC offers a liquid nitrogen level monitor, the 729A, that
can be used to protect the detector system against accidental warmups
which, in a HPGe coaxial detector system, may cause damage to the first
FET of the preamplifier, if the high voltage has not been removed.
This type of failure is more likely to occur in older systems. If
the high voltage is inadvertantly left on during a warmup, it is
possible that the increased internal pressure may cause a discharge
through the input FET.
The EG&G ORTEC 729A Liquid Nitrogen Level Monitor is a single-width
NIM module with audio and visual alarms that are activated when the
level of liquid nitrogen in the dewar falls below a predetermined
level. At the same time the module disconnects the high voltage supply
by remote shutdown. The module includes a battery that will activate
the alarms even in the case of a power failure.
The Gamma-X Coaxial Detector System, if disconnected from all
external electronics, can be warmed up, stored at room temperature
for any desired period, and then be cooled again. While this can be
done repeatedly without incurring damage, it is recommended that such
temperature cycling be kept to a minimum. As previously stated, before
the system is thermally cycled it must be disconnected from all external
electronics; high voltage can be applied to the detector only after a
minimum of six hours of cooling time. This is necessary each time
liquid nitrogen is introduced into the warm dewar. When a warmed-up
system is to be recooled, be careful to minimize contact between the
cold nitrogen gas and the cryostat, the preamplifier, or the high
voltage filter.

Gamma-X 5
1.4. SPECTROSCOPY WITH GAMMA-X DETECTORS
When any semiconductor detector is used in an energy spectroscopy
system, the detector element absorbs the energy from an incident
gamma ray or x-ray event and generates a current pulse whose integral
in the time domain has an amplitude that is proportional to the absorbed
energy. This pulse is processed through the preamplifier that is
attached to the detector assembly and then through the main amplifier
and into a multichannel analyzer or other data acquisition system.
The analyzer measures the amplitude of each pulse and stores a record
of the event in its memory. After a sufficiently large number of
events have been detected, measured, and stored, the data in the
multichannel analyzer represents a spectrum of energies vs frequency
of repetition.
60
For reference. Fig. 1.4 is a spectrum that was obtained for Co,
using a typical EG&G ORTEC Gamma-X Coaxial Detector. The range of
energies in this spectrum is from 0 to about 1.5 MeV and the two
principal energy lines for the source, at 1.17 and 1.33 MeV, are
seen clearly as high peaks in the spectrum. The vertical deflections
in the display are logarithmic rather than linear; this practice is
standard because it permits observation at the same time of small
peaks, resulting from relatively few counts, and major peaks that
result from very high count levels.
Figure 1.5 shows the portion of interest in the spectrum of
Fig. 1.4. Any energies outside these limits are simply not included
in the spectrum, but the details of the photopeaks are much easier to
discern in this partial spectrum.

Gamma-X
MMMW—
Fig. 1.4. Full Spectrim Taken with Fig. 1.5. Expanded Portion of Interest
EG&G ORTEC Gamma-X Coaxial Detector in the ®°Co Spectrum of Fig. 1.4
on ®^Co
As previously noted, contrary to all other types of coaxial Ge
detectors, the Gamma-X detector is a very useful spectroscopy tool
on X— or gamma—ray "energies down to below 10 keV. This feature is
discussed in more detail in Section 3.

Gamma-X 7
2. RECEIVING, UNPACKING, AND FILLING WITH LIQUID NITROGEN
2.1. GENERAL
Each EG&G ORTEC Gamma-X Coaxial Detector System is shipped in a
crate designed to protect all of the system elements. The systems,
shipped at room temperature, must be unpacked and then filled with
liquid nitrogen to be put into operation. In the side-looking
models, this is done easily and quickly by simply pouring the liquid
nitrogen from another dewar (See Section 2.2). A different procedure
is suggested in Section 2.2 for filling the dipstick models (Models
20 and 21).
2.2. UNPACKING AND FILLING WITH LIQUID NITROGEN
1. Remove any external strapping from around the crate. If
the crate has a heavy cardboard top, remove the screws that hold
the top to the base of the pallet. If the crate has a plywood top,
remove the six wing nuts that are accessible through the crate sides
near the floor. In either case, when the top cover is free from the
pallet base, lift it carefully straight up to uncover the detector-
preamplifier-dewar assembly. The system will look like that in Fig. 2.1.
2. Cut and remove the strapping that secures the cryostat and
preamplifier to the dewar handles. Inspect the aseembly for physical
damage that might have occurred during shipment.
3. Cut the straps that hold the dewar to the floor of the crate
and remove the complete detector assembly from the crate. Save the pad
under the dewar and use it as a shock mounting at its intended operating
location. Store the crate for possible re-use at a later date.

Gamma-X
'i SSiSmi
Fig. 2.1. A T3^ical Gamma-X Detector System, Showing
Method of Packing Inside the Crate
4. Fill the dewar with liquid nitrogen. For a side-looking
detector system, cover the detector and preamplifier with a sheet
of plastic or other protection to prevent any liquid nitrogen from
being accidentally spilled on these components. Then pour the liquid
nitrogen from a supply dewar into the operating dewar; keep top caps
in place on both dewars at all times except when the transfer is
actually being made so that moisture absorption and icing are minimized.
For a Model 20 or 21 system, a nitrogen transfer collar is
included between the top of the dewar and the cryostat assembly. The
two stainless steel tubes that project through the molded collar are
for filling and venting, and these are interchangeable. Attach a
length of tubing to the tube that will be used as a vent and allow it

Gamma-X 9
to hang over the side of the dewar to carry the cold gas and overflow
away from the preamplifier so that moisture condensation on and in its
components is prevented. Attach A short length (3 ft) of tubing from
the outlet of a filling dewar to the fill tube on the nitrogen transfer
collar of the detector assembly. To initiate the transfer, pressurize
the filling dewar with no more than 3 to 5 psig of dry nitrogen gas.
CAUTION
Provide a pressure relief at not more than
5 psig on the supply dewar so that it will
not become over-pressurized in the event that
the tubes should become blocked with ice.
For safety reasons, air or oxygen should not be used for pressurization.
Since the system is shipped at room temperature, initially the liquid
nitrogen will vaporize instantly as it cools the walls of the dewar and
the dipstick, and liquid nitrogen will be emitted from the vent tube
of the detector system. When the walls of the dewar and the dipstick
begin to be sufficiently cold, the liquid nitrogen will begin to collect
in the dewar. The transfer should continue until liquid nitrogen exhausts
from the vent tube. By then the vehement bubbling of the liquid nitrogen
in the dewar of the detector system should have largely subsided. Six
hours later, proceed to top off the dewar, thus replacing the liquid
nitrogen that will have been used to bring the system temperature down
to equilibrium. On older systems, there may be some condensation of
moisture on the end cap if a plastic bag of fresh silica gel desiccant
is not tightly in place, covering the end cap and cryostat.

10 Gamma-X
5. Although EG&G ORTEC does not reconmend this practice, it is
possible to fill and empty the dewar of a Model 20 or 21 system through
the dewar top when the dipstick has been removed. If this is done,
always hold the cryostat in its normal orientation (i.e., with the
end cap lower than the cooling rod); otherwise some molecular sieve could
fall down from the cooling rod into the end cap and result in
detector malfunction.
6. As previously stated, external electronics should be connected
to the detector system only when the detector has been properly cooled.

Gamma-X 11
3. CHECKOUT AND OPERATION
3.1. GENERAL
After the dewar has been filled to its capacity and the subsequent
steps have been taken as described In Section 2, Install the detector
In an energy spectroscopy system and operate the system to check the
specifications that are Included In the Quality Assurance Data Sheet
on page 111 of this manual. This will accomplish two purposes; assure
that no damage was Incurred during shipment and provide the basic
orientation to the use of radioactive sources for testing the system.
The basic specifications are based on spectral measurements made with
^^^Co and ^'^^Cd sources.
The electronic system to be used for the measurements Includes
the detector with Its attached preamplifier and high-voltage filter,
a main amplifier, an optional biased amplifier, a multichannel analyzer,
a precision pulse generator, a detector bias power supply, and an
oscilloscope. The typical system arrangement Is shown In Fig. 3.1.
DETECTOR
BIAS POWER
SUPPLY
PRECISION
PULSE
GENERATOR
n
HIGH
VOLTAGE
DETECTOR
PREAMPLIFIER
MAIN
OSCILLOSCOPE
1 ,
MULTICHANNEL
FILTER
AMPLIFIER
ANALYZER
OPTIONAL
BIASED
AMPLIFIER
Fig. 3.1. Interconnection of Components In a Typical Energy Spectroscopy Syst
em