New experimental data is presented which displays 137Cs resolution of both single Si(Li) devices and a detector stack 2 cm in height as a function of temperature (85 K greater than or equal to T greater than or equal to 245 K). We also discuss variations in photopeak shape which indicate that detector charge collection may be temperature dependent over the range of interest

Hubbard, G. Scott

Keller, R. G.

McMurray, Robert E., Jr.

Walton, J. T.

Wercinski, P. F.

Wong, Y. K.

English

NASA Technical Reports Server

1338 IEEE TRANSACTIONSON NUCLEAR SCIENCE, VOL. 41, NO. 4. AUGUST 1994
Variable Temperature Performance of a Si(Li) Detector Stack
G. Scott Hubbard, Robert E. McMurray, Jr., R.G. Keller, and P.F. Wercinski
NASA-Ames Research Center
Moffett Field, CA 94035-1000
J.T. Walton, Y.K. Wong
Lawrence Berkeley Laboratory
Berkeley, CA 94720 NASA-TM-III912
Abstract
New experimental data is presented which displays 137Cs
resolution of both single Si(Li) devices and a detector stack
2 cm in height as a function of temperature
(85 K <T <245 K). We also discuss variations in
photopeak shape which indicate that detector charge collection
may be temperature dependent over the range of interest.
I. INTRODUC_ON
We have previously reported laboratory experimental data
and Monte Carlo analysis for a Si(Li) gamma-ray detector
stack [1,2]. This configuration is intended to provide moderate
resolution [full-width at half-maximum (FWHM) 5-10 keV @
1 MeV)] over a broad temperature range (<90 K to 230 K).
This temperature range is consistent with the needs of
spaceborne applications using ambient (passive) cooling for
orbiting platforms _100 K) or for Mars surface operations
(130 K-230 K). Such an approach provides an attractive
alternative to the poorer resolution of scintillators (-60 keV
@ 1.3 MeV) and the cryogenic cooling requirements of HPGe
(< 100 K).
For simplicity in the earlier work, experimental resolution
data was collected at 175 K [1]. DC leakage current indicated
that acceptable device performance (FWHM < 10keV @
662 keV) could be expected up to -230 K, where leakage
current was - 10 hA. However, it is well known that leakage
current is only one contributor to detector performance, and it
has been suggested that detector performance often degrades as
a function of temperature faster than indicated by DC
measurements. In addition, the literature provides little
experimental data on the performance of Si(Li) detectors at
temperatures above that of liquid nitrogen (-77 K) [1]. This
current work addresses that absence of data, providing variable
temperature resolution measurements from 85 K to 245 K.
II. EXPERIMENTAL PROCEDURE
A. Device Testing
A total of 8 Si(Li) devices were tested in a variable
temperature cryostat constructed for this work. We evaluated 6
devices 5 mm thick and 2 devices 1 cm thick; all detectors
have a 3 cm 2 active area. These devices were fabricated using
standard lithium drifting techniques employed at the Lawrence
Berkeley Laboratory (LBL) silicon detector laboratory, with no
special attention to elevated temperature operation.
All devices feature a standard grooved structure, however
the 1 cm thick detectors had their exterior annulus reduced in
height to accommodate electrical connections to the center
active area more easily. We followed the convention of
applying positive bias to the Li n + contact, and obtaining the
detector signal from the Au Scbottky barrier side. In our
previous work, we typically fabricated detector stacks of four
5 nun Si(Li) devices to create an active absorption length of
2 cm. In this current work, we were able to obtain several
1 cm thick devices, which simplified the task of assembling a
2 cm stack. Future plans call for tests of a stack of four
1 cm devices.
Testing conditions were standardized except as noted
elsewhere. Both individual detectors and detector stacks were
biased to 600 volts and data collection times were
500 seconds. 137Cs (662 keV) was used throughout as the
"/-ray source. As noted in the previous work, this isotope was
selected because Compton scattering is the primary
mechanism of energy deposition for Si in this energy range
and therefore represents a rigorous test of the full energy
photopeak detection. As discussed later, an additional
parameter varied was umplitier shaping time.
B. Variable Temperature Cryostat Configuration
Figure 1 displays the elements of the cryostat
configuration. A key experimental challenge was to develop a
wide-range, variable temperature cryostat which is both low
noise and allows for rapid thermal cycling. Low noise
temperature control (90 K < T _<245 K) was achieved via
power input and control measurement at the cold finger. This
approach eliminated undesirable noise introduced when the
controller heater was placed near the detector cold plate inside
the vacuum shroud. Only temperature is measured at the
detector, using a calibrated Si diode. In order to reach higher
temperatures and also provide stable temperature control, we
employed a braid contact to the LN2 to provide a thermal
impedance to the cold sink. Rapid thermal cycling is achieved
0018-9499/94504.00 © 1994 IEEE
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1339
by submerging the cold finger in LN 2 until low temperature is
reached at the detector cold plate (85 K), then withdrawing the
apparatus until only the braid connects the cryostat with the
LN2.
The inset of Fig. 1 displays the internal detector mounting
scheme. We show spring loaded detector fasteners which were
constructed to provide good thermal contact through constant
pressure and also prevent breakage as metallic parts shrink at
low temperatures. Also visible is the aluminum 137Cs
source holder which guaranteed a constant detector/souree
geometry and also blocked the unwanted beta particle
continuum.
Figure 1. Experimental Apparatus. Insert shows detector
configuration in vacuum shroud.
C. Electronics
All measurements in this current work were conducted
using either the output from a single device or the summed
mode for detector stacks. As shown in Figure 2, the signal
output from the stack elements was combined from all devices
before the vacuum feedthrough to the preamplifier. The
spectroscopy amplifier, high voltage power supply and
mnltichannel analyzer (MCA) used are standard commercially
available units.
Selection and optimization of the preamplifier (provided by
LBL) was driven by the need to compensate for competing
noise sources across the broad temperature range of interest.
For example, the best electronic noise performance is typically
obtained by using a DC-coupled preamp, often with an opto-
feedback circuit and cooled FET. In our preliminary testing
we found that in the range of 150 K < T _<200 K leakage
currents became unacceptable for DC-coupled preamps.
Alternatively, AC-coupled preamps, which are far more
tolerant of high leakage currents, are customarily designed for
high capacitance surface barrier devices used for charged
particle spectroscopy and therefore not optimized for a SiCLi)
stack capacitance of ~10 pf. Our choice for the current
experiments was an AC-coupled preamp with input circuit and
FET selected for the lower capacitance and leakage current
varying from a few pA to >50 nA. In future work we believe
more improvement in the electronic noise component is
possible.
.--I HIOH-VObTAOE I
rRE-_ ^_ ^oc,! [
1 oloDE --
_ I TF-_II_ERATURE
l I_NS°R
1
Figure 2. Signal Processing and Temperature Controllers.
Control heater and diode are mounted on cold finger
as shown in Figure 1.
III. RESULTS AND DISCUSSION
A. Initial Device Testing and Selection
To establish suitability of Si(Li) devices over the range of
interest, and to provide a basis of comparison with our
previous leakage current measurements, five Si(Li) devices
were selected on the basis of room temperature noise
performance from a larger group of detectors. (A sixth detector
was discarded on the basis of excess leakage at ambient.)
Results of 137Cs resolution measurements vs. temperature
for all these detectors ate shown in Figure 3. Electronic and
environmental noise performance was not optimized, ralher the
results were used for qualitative screening only. As can be
seen, the resolution variation is relatively small (+20%)
among devices up to temperatures of approximately 200 K.
In addition, such detectors individually met the key resolution
criteria of <10 keV up to 230 K. Based on these results we
concluded that even without special fabrication or selection
procedures, devices on hand at LBL should yield stacks which
respond consistently to variable temperature operation.
B. Detector Stack Results
As noted earlier, several 1 cm thick devices were available,
which simplified the construction of a 2 cm stack. This
absorption length was chosen initially as appropriate to test
rigorously the stack concept using a 662 keV source (137Cs).
Figure 4 shows the resolution (FHWM) of 137Cs photopeak
vs. T for the 2 cm stack. (Amplifier shaping time was
optimized throughout data collection - see Sec. III C).
1340
15
> 11
... i.,. i ... i... i.,. i... i--- i-..
.................-.. ..i ..
i _,__#7268 i i i i ?]r
........"- #7098 ....i .........- .........- .........i"Y ....
i[ -'-#3916 i i i "I/
........_"I---+-#3982....! .......-i.........+--_,_......
iiii
90 130 170 210 250
Temperature, K
Figure 3. Resolution vs. temperature at 662 keV, five test
detectors.
Data analysis was conducted with standard MCA output and
an off-line ganssian fitting routine. It was found that analysis
of the data using a gaussian peak fit yielded somewhat more
consistent results than the commercially provided FWHM
measurement algorithm resident in the MCA. The results
displayed in Figure 4 were fitted with a constant background
superimposed with an exponential curve. A detector stack
resolution of 3.92 keV up to 150 K was obtained through
this analysis. Electronic noise as determined by precision
pulser is 3.68 keV for the same temperature range. If one
assumes that noise contributions sum in quadrature, one can
determine that the contribution of the Si(Li) device is
_/(3.92keV) 2 - (3.68keV) 2 = 1.4 keV.
This value is approximately the same as the noise of charge
production statistics in Si at an energy of 662 keV and a Fano
factor of 0.15. At higher temperatures this relationship does
not hold. These results suggest that "normal" Si(Li) device
performance may be obtained up to -150 K, but that an
excess noise mechanism begins to dominate at higher
temperatures. A measurement of the DC leakage current of
this stack from 100 K to 240 K demonstrates that the current
remains below 1 nA up to 220 K. The noise contribution
from this current is small compared to the measured FWHM.
Further evaluation of both DC and AC components of this
noise contribution is underway.
C. Optimization of Shaping time.
During testing and evaluation of both individual devices
and detector stacks, asymmetric 662 keV photopeaks were
observed at elevated temperatures. Figure 5 a-d shows a
typical variation in peak shape vs. increasing T for a stack of
four, 5 mm detectors for fixed amplifier shaping times (Xs).
The asymmetry was consistently observed to be characterized
by counts which accumulated as the well-known "tail" on the
lower energy side of the peak.
16
14 ........_........." ........._........._.........i..........i..........i........ '
_12 | ....,IS137"Csb'WHMIPulser FWHM I........................i i
lO -.--: . -_i I,-.._.........._..................
8 ......... 4 ......... 4 ......... _-......... _-......... i.......... _--. --o4 .......6
4 ...._-_.. _'" _!
,,,i,..l.,, I,,.I...I... I*,,I..,
90 130 170 210 250
Temperature(K)
Figure 4. FWHM resolution of 137Cs photopeak using a
Si(Li) stack 2 cm in height, x s was adjusted for
optimum peak shape at each temperature
As shown in Figure 6 a-c, a symmetrical gaussian peak
shape could be restored by using progressively longer Xs. The
results in Figure 6 were obtained from a single detector, but
are consistent with all stack measurements. Such device
behavior is generally associated with charge trapping centers
which produce so-called "slow" pulses. These slow pulses can
then be collected into the full-energy photopeaks by
lengthening the amplifier shaping time.
For the Si(Li) stack results shown in Fig. 4, optimum
resolution was obtained by adjusting the amplifier shaping
time as a function of temperature. Figure 7 shows the
resolution of the 2 cm stack as function of amplifier shaping
time for -85 K and -200 K. In this figure, the fitted curves
shown are only an aid to the reader; no functional relationship
is implied. As can be seen, the optimum Xs for minimum
FWHM shifts by a factor of >2. During most measurements
over the entire temperature range, only three different Xs values
were required: 1.5 lxS, 3 _tS and 5 IxS. For a given Xs, very
little peak shift was observed.
1341
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6OO
5OO
4OO
E
3
o 3000
2OO
IO0
0
60O
Figure 5(a)
I .... I .... T.... i .... ! .... T.... i .... I ....
.........i l T=lO2Kl i.........L i..........
F........._-----i r----"........_'--_,_ .........i i i i := oi i[
i :: i i i ol :: "I
I.........i .........i.........._.........._........t " ot. ,i .........! i _ i o i o!
.........i ..........[.........i.........i.........-....e...i.......°-i.........
610 620 630 640 650 660 670 680
Energy(keV)
7OO
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¢/}
"E 4OO
o 300O
200
100
0
600 610
Figure 5(c)
...................................................... , .......... t ....................
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..................... t .......... _.......... ,.......... :.......... ."....................
.......... _.......... _.......... _.......... _.......... _....... _...... _.._ .........
i i i i o i i ,
620 630 640 650 660 670 680
Energy(keY)
700
.... i .... i .... i .... i .... i .... I .... i ....
: i :
sot .........[.........._ ........................i _ i..........
i i i ! ! i
sot ..........i1T=170.5KI..........i.......... i ......... J ..........
i l o.2s_secI '_ '_ _
",_ 400 ..........;I ! ..................._---_---i..........
: i i_'oi :: :
: : o" o : '
200
.........._ .......:, ....... ......., ......._ ..°!....... ........
100 i i i ii ._
o 'iTiiTi 
600 610 620 630 640 650 660 670 680
Energy(keV)
Figure 5(b)
700
600
500
--_ 400
¢.
8 300
200
100
0
600 610
Figure 5(d)
I .... ! .... ! .... ! .... I .... l .... f .... ! ....
i i i ! i i i
........ ;.......... ;.......... ; ..................... E.......... " .......... .".........
.........!lil T=210K ..........i..........i ..........!.........
il 0.25psec i ! i
........................................... ; .......... :.......... , .......... [........
....................i i i i ! ,ii.............. ........1
620 630 640 650 660 670 680
Energy(keV)
Figure 5. Degradation in photopeak shape as a function of
temperature for a single shaping time (0.25 pS).
Stack of four 5 mm thick devices selected from
those shown in Figure 3.
IV. SUMMARY AND CONCLUSIONS
In summary the Si(Li) stack measured exhibits the required
resolution over the temperature range of interest. We have
observed a FWHM for 662 keV of ___5 keV for T < 200 K
and < 10 keV for T < 230 K. To obtain an optimized peak
shape requires longer Xs as device temperature increases. As
described earlier, the most likely explanation for this
phenomenon is that regions of non-uniform charge collection
appear at elevated temperatures and result in charge trapping.
Extending x s allows "slow" trapped charge to be collected in
the full energy photopeak.
Based on results to date, further improvements are possible
in detector noise performance. Examples of these are
refinements in preamplifier FET selection and input circuit.
In addition, enhancements in the device charge collection may
be realized though modifications to the detector processing.
For example, fabrication and test of Si(Li) devices using a new
phosphorus diffusion technique may remove charge trapping
centers [3]. To fully understand the trapping which appears as
a function of temperature we plan to examine the Si(Li) stacks
with an analytical technique such as Deep Level Transient
Spectroscopy.
In order to demonstrate spectrometer performance, we plan
to readout individual device signals from the stack elements
and apply previously developed detection algorithms to
measurement of multi-line spectra [2].
1342
00 ....
i................i....i....i....160 ......... •.......... _.......... :.......... :.......... :.......... :.......... ,,,..........il T=230K! i i i
o [ _ 6o0
0 80 f ! :°
........ _.......... :.......... _.......... :.......... _......... _,._. .............
i i i *,i °:
i i ! i i #i *,i
40 ........i..........i..........i .........i ......._,,_t_-_----o-i..........
: ' :0 : 0 _- _ : 0 1
0 _/i'"_' i .... i .... i .... =,.... ,'_
600 610 620 630 640 650 660 670 680
Energy(keV)
Figure 6(a)
00 .... i .... i .... i .... ; .... i .... v .... v .....
i
160 i :
T=230K
120 ' ......... 0.51LtSeC ............................. ,.........
o i
0 80 ............................. ,':................... .-............... ° .............
40 .............................+ i......_+;i .........
! i o_ o_ "
0 _..,,_:.:._.,,--_....i .... ,...
600 610 620 630 640 650 660 670 680
Energy(keV)
Figure 6(b)
200
#' i
il ''T=230K _'0 i.
10o .....................
o i •
O
ol
O : O
so ..........................................i.........._ ei _,_ .......
i
: 0 , : 0 ; 0 i,..o
600 610 620 630 640 650 660 670 680
Energy(keV)
Figure 6(c)
, , . m I I I i • , , [ I I , i - . , ] , . I i . I I
i i i i ; .
7 ........... !._...,.....i ........... i............ _........... i ........ i...........
g6
5 ........... i................
: i 9 ; ; ;
4 ....._ _-_---,-.-T=200.4K _.....
//:° "-I Io o i i i i !3 ._.,.i,..i...
0 2 4 6 8 10 12 14
Shaping Time (lasec)
Figure 7. FWHM vs. amplifier shaping time for a 2 cm
Si(Li) stack. Optimum Xs for minimum FWHM
shifts by >2 times from 85 K to 200 K. Curves
are shown as an aid to the reader only.
V. REFERENCES
(1) G. Scott Hubbard, et al, IEEE Trans. Nuclear Science Vol. 39,
Proceedings of IEEE 1991 NS Symposium, pp. 981-986
(1992).
(2) Robert E. McMurray, Jr., et al, IEEE Trans. Nuclear Science
Vol. 40, Proceedings of IEEE 1992 NS Symposium, pp. 882-
889 (1993).
(3) J.T. Walton, N. Derhacobian, Y.K. Wong, and E.E. Hailer,
Applied Physics Letters, in press.
Figure 6. 137Cs photopeak shown for a single temperature,
as a function of Xs. Single detector # 6454.



Variable Temperature Performance of a Si(Li) Detector Stack

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