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High-energy radiation effects on the isophot far-infrared detectors
in/rure
0020-08Qt/90$3.00+ 0.00 Copyright 6 1990Pergamon Press pk
RESEARCH NOTE HIGH-ENERGY
RADIATION FAR-INF~RED
EFFECTS ON THE ISOPHOT DETECTORS
J. BLUM*, CH. HAJDUK, D. LEMKE,A. SALAMA~ and J. WOLF Max-Planck-lnstitut
fiir Astronomie, Heidelberg, Federal Republic of Germany
(Received 21 March 1989; in revised form 25 April 1989) Abstract-The performance degradation of far-infrared detectors subjected to y-radiation was measured for various operating conditions. Responsivity drifts incompatible with precise photometry were observed. Curing procedures are required for flight operations.
In laboratory simulations of the radiation environment expected to be experienced aboard the Infrared Space Observatory (ISO), (I) bulk ge~ani~ extrinsic photoconductors (t~i~lly 1 x 1 x 2 mm3) developed for the ISOPHOT inst~ment(‘) have been subjected to y-radiation from a “Co source. A dose rate of 80 mrad/h is representative of a detector shielded by the IS0 cryostat while passing the electron belts. The electrons are stopped in the cryostat walls emitting bremsstrahlung y-photons. (2) Our experimental findings concerning spike rate, changes in the responsivity to IR signals, and detector noise for various operating conditions are summarised in Table 1. Below we shall address these effects one by one in more detail. y-Induced ionisation events are observed as spikes in the detector current. in the case of Ge:Ga several detector geometries with the same material could be compared. The spike rate scales roughly with the detector’s volume. Preliminary histograms of the charge liberated in spike events show an approximate exponential fall-off towards larger spikes. In operations, the frequent small spikes will be difficult to discriminate against, and they will result in an increased noise level. The variations of the current responsivity induced by the relatively low dose rate of 80 mrad/h investigated ranges from 2-3% for Ge:Be over factors of 3-4 for Ge:Ga to factors of 20-30 in the case of stressed Ge:Ga. It is as yet unknown whether or not these results will similarly apply to the high dose rates, typically 4 rad/h, expected during proton belt passage. Figure 1 illustrates the behaviour of the stressed Ge:Ga detector in more detail. When the y-source is installed, the responsivity increases rapidly to reach an equilibrium level after about 3 h. When the y-source is removed again, the responsivity slowly drops; complete recovery could not be observed within the limited measurement time. The equilibrium level of the responsivity depends strongly on the bias voltage, the dose rate and the infrared background. As Fig, 1 illustrates, a reduction of the bias voltage r&luees the changes in responsivity. However, operation of detectors at reduced bias is not a viable option, as maximum sensitivity in a clean environment is achieved for 60 mV bias at 2 K, while at a lower bias the detectors performance is much poorer from the beginning. Figure 2 reveals that the responsivity enhancement scales roughly with the square root of the dose rate. Under increased IR background the responsivity variations are suppressed and the recovery after termination of y-irradiation is accelerated, compare Table 1. A similar effect would be expected from an increased operating temperature, a small reduction of the responsivity variations at increased temperatures is indeed observed in Fig. 1. The evolution of detector noise over the course of y-irradiation is presented in Fig. 3. Under irradiation the noise level is exaggerated by the spikes, since no spike discrimination was attempted, *Present address: Max-Pianck-Institut tESA Fellow.
fur Kernphysik, Heidelberg, Federal Republic of Germany.
93
8.5 x 5.5 x 9.0 x 5x 5 x 5x 5x 5 x 5x 2.5 x 9.5 x
2.6 x 2.6 x 2.6
1.1 x I.1 x 1.4
Ge:Ga DS 41 (Battelle)
Stressed Ge:Ga GGSIOI (Battelle)
*I,,, = r(R = l/2 R,,) after yaw l * V,,,, = 0 during irradiation.
2.4 x 10s
2.5 x 2.5 x 1.0
Ge:Ga IRL 102 (Infr. Lab.)
10’ 108 10’0 10s 108 108 10s 10s 10s IO’ 109
3.6 x IO” 4.9 x 10’0
IO’ log 108 10s 10s
2.5 x 2.5 x 1.0
x x x x x
Ge:Ga IRL 101 (Infr. Lab.)
9.9 1.4 3.5 4.1 9.7
(Phcr%‘)
1.0 x
1.0 x 1.0
Volume (mm’)
3.2 3.2 3.2 2.0 2.5 2.0 2.0 2.0 2.0 2.0 2.0
3.2
3.0 3.0
3.0 3.2 1.9 2.9 2.8
0.6 0.6 0.6 0.060 0.060 0.030 0.015 0.030 0.030 0.030 0.030
0.1
0.1 0.1
-1.3 -2 -3.5 + 2.4 -1.3 -100 -100
4.4 2.1 3.4 3.1 2.4 3.511 II 7.1 1.3 32 29 16 10 II I 15 11
0.05 0.06 0.02 0.06 0.12 0.08 0.08 0.08 0.08 0.08 0.08 0.04 0.02 0.08 0.08
2.5 1.5 2.5 I .o 4.5 4.4 2.0 3 3 3 3 3 3 3 3
6
33 24 8.4 6. .I6 5...18 6. .3 3... 1.2
-100
-17 -100
0.08 0.08
-25
0.76 1
< 3% < 2% < 1% 0.98 AR/R< 1%
AR/R AR/R AR/R
Responsivity change Rl%
0.08 0.04 0.08 0.08 0.08
Dose rate (rad h-‘)
2.35 3.2 1.2 2.2 1.2
‘tan (h)
NEP/NEP, during y-irradiation (with spike)
0.4 0.4 0.3 0.4 0.6
8 2
60 75 25 IS 280 200 200 20 20 20 20 10 5 20 20
1.2 0.8
loo 90
40
Spike
(R) and noise
1.15 5 18 I 0.2
0.3
1.06
0.84
I .32 1.46
NEP/NEP, immediately after y-irradiation
of FIR detectors under y-irradiation for different operating conditions: temperature (T), bias voltage (Vs) IR photon background (Qs). Responsivity equivalent power NEP are quoted relative to their pre-irradiation values Rs and NW,,. f,aa is the length of the irradiation period
Ge:Be K9C11 (Battelle)
Detector
Table 1. Behaviour
95
Research Note
.
++++
b0 10
d
-
R/R0
+
.O 0
S-
0, 0
3-
.O
+
+ +x
+x x
$+
x
xxx
x
xx
x XxXx
x
x
+ xx
.
o”+ 2-
++ ++
+ * ++
x
uslo, = 60mV.T:
o us,,,, ~60 + uB,_
2.0K
mV, Ts 2.6 K
: 30mV.
T-2.0
K
x Us,o~=1SmV,T:2.0K
l-
I
tY
01
0”
I
I
I
I
0
1
2
3
Time
(h)
(b)
10
-
R/R0
2
1 y off I 01
T I
I
I
I
0
1
2
3
Time
lh)
Fig. I. Responsivity increase (a) and decrease (h) during and after y-irradiation for the stressed Ge:Ga detector under different operating conditions.
and an order of magnitude over quiet level. The noise pedestal, present immediately after removing the y-source, indicates the increased intrinsic noise and poses a fundamental limit for the efficiency of spike discrimination procedures. Under nominal operating conditions ( VB= 60 mV, T = 2 K) the noise pedestal is 5 times the pre-irradiation value and takes about an hour to decay. For the low bias measurements this effect is masked, since the preamplifier noise contribution to the noise equivalent power dominates here and is reduced by the increased responsivity. Responsivity variations as reported here are incompatible with accurate photametry. Variations in dose rate along the orbit and in the infrared flux, depending on the celestial targets, can together
96
Research Note
.--7
0
bK 0:' P
0.5
-
+
cc! o-
. I
I
I
0
0.5
1.0
b/b,,,
Fig. 2. Dependence of the responsivity saturation level for the detector from Fig. 1 on the irradiation dose rate d. V,,,, = 30 mV, T = 2 K, d,, = 0.08 rad h-l, R-/R, = 18.
str GGS llrl4mm~
Ge GO Ilr
101
Ge Go ,t,. .-
GGS
101
llxIl~l.4mm~
Ve,., = 60 m”
. *
T120K
. .
.
-kb
‘t
5. . . . . . .. . . .
. .
I 1
A-
I
2
3
t
1
IO-”
2. -
I’ -.. I
.
I
1
I
I
2
3
17O’f 4
.
.. *a .
Fig. 3. NEP variations under y-irradiation
.
,..
. .
. .--
.&e+*‘*”=*
t
Y tron
I
0
Time
*
I
3
IS in”
T’20K
i
--;.. . ...
l”O.
I 5
Gc Ga str GGS 101 11 x 1.1 x 1.4 mm3
%?a'.
0
1 4
3
ug,.,=
-...,.........
.m.
*roff
I 2
U~,.r=30mV
* .’
k
t
, I
Ge Go str GGSlOl 11xll*l.4mm3 T’20K
l
tron 0
I 1
I 2
, 3
tr
l
Off
, 4
I 5
(hl
for the stressed Ge:Ga detector under different operating conditions.
produce unpredictable changes in flux calibration. Hence operational procedures for detector curing to restore the responsivities have to be developed and are currently under investigation. Acknowledgemenr-This
work was supported by the BMFT under grant OlQI85016.
REFERENCES 1. Proc. SPIE p, 589, Papers 589-28 and 589-31 (1985); Proc. SPIE 1130. In press. 2. E. J. Daly, ES.4 J. 12, 229 (1988); E. J. Daly et al., ESTEC internal note WMA/ISD/l50/7 (1987). 3. H. Aumann, B. Brown, F. Gilktt, W. Irace, D. Langford, P. Mason and R. Salazara, Infrared astronomical satellite, in orbit performance assessment. JPL D-871 (1983).
0020-08Qt/90$3.00+ 0.00 Copyright 6 1990Pergamon Press pk
RESEARCH NOTE HIGH-ENERGY
RADIATION FAR-INF~RED
EFFECTS ON THE ISOPHOT DETECTORS
J. BLUM*, CH. HAJDUK, D. LEMKE,A. SALAMA~ and J. WOLF Max-Planck-lnstitut
fiir Astronomie, Heidelberg, Federal Republic of Germany
(Received 21 March 1989; in revised form 25 April 1989) Abstract-The performance degradation of far-infrared detectors subjected to y-radiation was measured for various operating conditions. Responsivity drifts incompatible with precise photometry were observed. Curing procedures are required for flight operations.
In laboratory simulations of the radiation environment expected to be experienced aboard the Infrared Space Observatory (ISO), (I) bulk ge~ani~ extrinsic photoconductors (t~i~lly 1 x 1 x 2 mm3) developed for the ISOPHOT inst~ment(‘) have been subjected to y-radiation from a “Co source. A dose rate of 80 mrad/h is representative of a detector shielded by the IS0 cryostat while passing the electron belts. The electrons are stopped in the cryostat walls emitting bremsstrahlung y-photons. (2) Our experimental findings concerning spike rate, changes in the responsivity to IR signals, and detector noise for various operating conditions are summarised in Table 1. Below we shall address these effects one by one in more detail. y-Induced ionisation events are observed as spikes in the detector current. in the case of Ge:Ga several detector geometries with the same material could be compared. The spike rate scales roughly with the detector’s volume. Preliminary histograms of the charge liberated in spike events show an approximate exponential fall-off towards larger spikes. In operations, the frequent small spikes will be difficult to discriminate against, and they will result in an increased noise level. The variations of the current responsivity induced by the relatively low dose rate of 80 mrad/h investigated ranges from 2-3% for Ge:Be over factors of 3-4 for Ge:Ga to factors of 20-30 in the case of stressed Ge:Ga. It is as yet unknown whether or not these results will similarly apply to the high dose rates, typically 4 rad/h, expected during proton belt passage. Figure 1 illustrates the behaviour of the stressed Ge:Ga detector in more detail. When the y-source is installed, the responsivity increases rapidly to reach an equilibrium level after about 3 h. When the y-source is removed again, the responsivity slowly drops; complete recovery could not be observed within the limited measurement time. The equilibrium level of the responsivity depends strongly on the bias voltage, the dose rate and the infrared background. As Fig, 1 illustrates, a reduction of the bias voltage r&luees the changes in responsivity. However, operation of detectors at reduced bias is not a viable option, as maximum sensitivity in a clean environment is achieved for 60 mV bias at 2 K, while at a lower bias the detectors performance is much poorer from the beginning. Figure 2 reveals that the responsivity enhancement scales roughly with the square root of the dose rate. Under increased IR background the responsivity variations are suppressed and the recovery after termination of y-irradiation is accelerated, compare Table 1. A similar effect would be expected from an increased operating temperature, a small reduction of the responsivity variations at increased temperatures is indeed observed in Fig. 1. The evolution of detector noise over the course of y-irradiation is presented in Fig. 3. Under irradiation the noise level is exaggerated by the spikes, since no spike discrimination was attempted, *Present address: Max-Pianck-Institut tESA Fellow.
fur Kernphysik, Heidelberg, Federal Republic of Germany.
93
8.5 x 5.5 x 9.0 x 5x 5 x 5x 5x 5 x 5x 2.5 x 9.5 x
2.6 x 2.6 x 2.6
1.1 x I.1 x 1.4
Ge:Ga DS 41 (Battelle)
Stressed Ge:Ga GGSIOI (Battelle)
*I,,, = r(R = l/2 R,,) after yaw l * V,,,, = 0 during irradiation.
2.4 x 10s
2.5 x 2.5 x 1.0
Ge:Ga IRL 102 (Infr. Lab.)
10’ 108 10’0 10s 108 108 10s 10s 10s IO’ 109
3.6 x IO” 4.9 x 10’0
IO’ log 108 10s 10s
2.5 x 2.5 x 1.0
x x x x x
Ge:Ga IRL 101 (Infr. Lab.)
9.9 1.4 3.5 4.1 9.7
(Phcr%‘)
1.0 x
1.0 x 1.0
Volume (mm’)
3.2 3.2 3.2 2.0 2.5 2.0 2.0 2.0 2.0 2.0 2.0
3.2
3.0 3.0
3.0 3.2 1.9 2.9 2.8
0.6 0.6 0.6 0.060 0.060 0.030 0.015 0.030 0.030 0.030 0.030
0.1
0.1 0.1
-1.3 -2 -3.5 + 2.4 -1.3 -100 -100
4.4 2.1 3.4 3.1 2.4 3.511 II 7.1 1.3 32 29 16 10 II I 15 11
0.05 0.06 0.02 0.06 0.12 0.08 0.08 0.08 0.08 0.08 0.08 0.04 0.02 0.08 0.08
2.5 1.5 2.5 I .o 4.5 4.4 2.0 3 3 3 3 3 3 3 3
6
33 24 8.4 6. .I6 5...18 6. .3 3... 1.2
-100
-17 -100
0.08 0.08
-25
0.76 1
< 3% < 2% < 1% 0.98 AR/R< 1%
AR/R AR/R AR/R
Responsivity change Rl%
0.08 0.04 0.08 0.08 0.08
Dose rate (rad h-‘)
2.35 3.2 1.2 2.2 1.2
‘tan (h)
NEP/NEP, during y-irradiation (with spike)
0.4 0.4 0.3 0.4 0.6
8 2
60 75 25 IS 280 200 200 20 20 20 20 10 5 20 20
1.2 0.8
loo 90
40
Spike
(R) and noise
1.15 5 18 I 0.2
0.3
1.06
0.84
I .32 1.46
NEP/NEP, immediately after y-irradiation
of FIR detectors under y-irradiation for different operating conditions: temperature (T), bias voltage (Vs) IR photon background (Qs). Responsivity equivalent power NEP are quoted relative to their pre-irradiation values Rs and NW,,. f,aa is the length of the irradiation period
Ge:Be K9C11 (Battelle)
Detector
Table 1. Behaviour
95
Research Note
.
++++
b0 10
d
-
R/R0
+
.O 0
S-
0, 0
3-
.O
+
+ +x
+x x
$+
x
xxx
x
xx
x XxXx
x
x
+ xx
.
o”+ 2-
++ ++
+ * ++
x
uslo, = 60mV.T:
o us,,,, ~60 + uB,_
2.0K
mV, Ts 2.6 K
: 30mV.
T-2.0
K
x Us,o~=1SmV,T:2.0K
l-
I
tY
01
0”
I
I
I
I
0
1
2
3
Time
(h)
(b)
10
-
R/R0
2
1 y off I 01
T I
I
I
I
0
1
2
3
Time
lh)
Fig. I. Responsivity increase (a) and decrease (h) during and after y-irradiation for the stressed Ge:Ga detector under different operating conditions.
and an order of magnitude over quiet level. The noise pedestal, present immediately after removing the y-source, indicates the increased intrinsic noise and poses a fundamental limit for the efficiency of spike discrimination procedures. Under nominal operating conditions ( VB= 60 mV, T = 2 K) the noise pedestal is 5 times the pre-irradiation value and takes about an hour to decay. For the low bias measurements this effect is masked, since the preamplifier noise contribution to the noise equivalent power dominates here and is reduced by the increased responsivity. Responsivity variations as reported here are incompatible with accurate photametry. Variations in dose rate along the orbit and in the infrared flux, depending on the celestial targets, can together
96
Research Note
.--7
0
bK 0:' P
0.5
-
+
cc! o-
. I
I
I
0
0.5
1.0
b/b,,,
Fig. 2. Dependence of the responsivity saturation level for the detector from Fig. 1 on the irradiation dose rate d. V,,,, = 30 mV, T = 2 K, d,, = 0.08 rad h-l, R-/R, = 18.
str GGS llrl4mm~
Ge GO Ilr
101
Ge Go ,t,. .-
GGS
101
llxIl~l.4mm~
Ve,., = 60 m”
. *
T120K
. .
.
-kb
‘t
5. . . . . . .. . . .
. .
I 1
A-
I
2
3
t
1
IO-”
2. -
I’ -.. I
.
I
1
I
I
2
3
17O’f 4
.
.. *a .
Fig. 3. NEP variations under y-irradiation
.
,..
. .
. .--
.&e+*‘*”=*
t
Y tron
I
0
Time
*
I
3
IS in”
T’20K
i
--;.. . ...
l”O.
I 5
Gc Ga str GGS 101 11 x 1.1 x 1.4 mm3
%?a'.
0
1 4
3
ug,.,=
-...,.........
.m.
*roff
I 2
U~,.r=30mV
* .’
k
t
, I
Ge Go str GGSlOl 11xll*l.4mm3 T’20K
l
tron 0
I 1
I 2
, 3
tr
l
Off
, 4
I 5
(hl
for the stressed Ge:Ga detector under different operating conditions.
produce unpredictable changes in flux calibration. Hence operational procedures for detector curing to restore the responsivities have to be developed and are currently under investigation. Acknowledgemenr-This
work was supported by the BMFT under grant OlQI85016.
REFERENCES 1. Proc. SPIE p, 589, Papers 589-28 and 589-31 (1985); Proc. SPIE 1130. In press. 2. E. J. Daly, ES.4 J. 12, 229 (1988); E. J. Daly et al., ESTEC internal note WMA/ISD/l50/7 (1987). 3. H. Aumann, B. Brown, F. Gilktt, W. Irace, D. Langford, P. Mason and R. Salazara, Infrared astronomical satellite, in orbit performance assessment. JPL D-871 (1983).