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A low-noise charge-sensitive preamplifier for an X-ray detector of a Nb-based superconducting tunnel junction
NUCLEAR INSTRUMENTS P METHODS IN PHYSICS RESEARCH
. __ __ fii!B ELSEYIER
Nuclear
A low-noise
Instruments
Secl~on A
and Methods in Physics Research A 401 (1997) 299-308
charge-sensitive preamplifier for an X-ray detector a Nb-based superconducting tunnel junction
M. Ukibe”*b**, M. Kishimoto”,
M. Katagiri”,
M. Kurakadoc,
of
M. Nakazawa”*b
“Advanced Suence Reseawh Center. Japarl Atomic Energ? Research Imtitute, Tokai-mura. Naka-gun. Iharaki-ken. bDepartnlent of Quantum Engirwwing artd ~Istems Science. Utziwmit~, qf Tohyvo, Tobo, Japan ‘Advanced Trchnologv Research Lahomtories, Nippon Steel Corporatiorl, Japan
319-l 1. Japan
Received 26 May 1997; received in revised form 15 July 1997
Abstract A low-noise charge-sensitive preamplifier with cooled 4-JFETs was developed for an X-ray detector of a Nb-based superconducting tunnel junction with a large capacitance and a low leakage current. The low-noise characteristics for a wide range of shaping times were obtained by using a selected JFET, 2SK190 in the first stage of the preamplifier. A new formula to simulate the preamplifier noise was developed and applied to the optimization of preamplifier design. A high-energy resolution of 66 eV for 5.9 keV X-ray and 51 eV for pulser signals was obtained by using the optimized preamplifier with an X-ray detector of a 178 pm x 178 pm Nb/Al-AlO,/Nb superconducting tunnel junction with a junction capacitance of 1900pF and a bias current of 1.44nA. Ke~vmrls:
High-energy
Charge-sensitive resolution
preamplifier;
Cooled
JFET;
X-ray detector;
1. Introduction A radiation tunnel times
junction
detector
higher-energy
a semiconductor
using
(STJ) is expected
a superconducting to have
resolution detector
than
because
has a small energy gap of meV that is much smaller than that ductors. X-ray detectors with STJs therefore, investigated in many groups superconductor
about that
the
40 of
metallic
in the order of semiconhave been, in order to
*Corresponding author. Tel: + 81 29 282 6080; fax: + Sl 29 282 6122: e-mail; [email protected]
0168-9002./97/$17.00 #?I 1997 Elsevier Science B.V. All rights reserved PI1 SO168-9002(97)00967-4
Nb-based:
Superconducting
tunnel junction;
realize X-ray spectroscopy with good energy resolution, but the theoretically expected resolution has not been achieved yet. Hence, the approach to improve the energy resolution of STJ X-ray detectors is still of a challenging one. There are two main approaches to improve the energy resolution of STJ detectors. One is the utilization of trapping effect in an STJ in order to efficiently collect electrons (i.e. quasiparticles) produced by X-ray absorption in a superconductor [l]. By using an Al layer for trapping the quasipartitle. Mears et al. could get electrons in their STJ detector as many as ten times more than by a Nb layer [2]. In this case, the characteristics of the
tunnel junction become generally worse, namely, a low dynamic resistance, a large leakage current and a long charge collection time resulting due to the small energy gap of an Al superconductor as compared with that of a Nb superconductor. Hence, they applied a current-sensitive preamplifier using SQUIDS to deal with electron signals produced in the STJ because the influence of the low dynamic resistance and the large capacitance of the STJ on the noise performance of the SQUID is negligibly small [3,4]. They obtained a FWHM energy resolution of 29 eV for 5.9 keV X-ray by an 100 pm x 100 pm Nb/Al,/AlO,/Al/Nb STJ having 200 nm Al trapping layers on both sides of the tunnel barrier at 0.2 K. Another approach is the improvement of the STJ characteristics by fabricating a perfect STJ structure by which a low leakage current. a small diffusion effect for the quasiparticles produced by X-rays, a constant charge collection time, etc., result [S]. The great advantage of this approach is that X-ray spectroscopy with high counting rate can be realized because the short charge collection time less than several ps is reserved. In this case. a charge-sensitive preamplifier using Junction-type Field Effect Transistors (JFETs) is adopted with this STJ in a similar way as that for a semiconductor detector. Here the optimal design of the preamplifier being adaptive to a large capacitance of the STJ is an essential problem. We attempted to improve the energy resolution of the Nb-based STJ detector having a low leakage current by the latter approach. In this paper, we describe a charge-sensitive preamplifier optimized for the STJ, a formula for simulation of the noise characteristic and X-ray detection with an excellent energy resolution by the optimized preamplifier in the following sections.
2. Experiment
for optimization
of the preamplifier
It is necessary to optimize the charge-sensitive preamplifier for an STJ having a large capacitance in order to develop a high-resolution X-ray detector. It is preferable that the optimized preamplifier has good noise performance at a long shaping time of the spectroscopy amplifier because the
effective quasiparticle lifetime in the STJ becomes longer if the crystallization of the superconductor layer in the STJ is perfect [6--81. Since the capacitance of an STJ is much larger than that of semiconductor detectors, it is necessary to increase the transconductance of the JFET used for the first stage in the preamplifier. Usually. a cooled JFET having a large transconductance has been used to decrease the preamplifier noise resulting from a large capacitance of the STJ [5,9%12]. Also, parallelly connected JFETs have been used for this purpose though they dissipate more power [l 11. On the other hand, an excellent energy resolution is obtained with a long shaping time, when the JFET has low noise characteristics at frequencies less than 10 kHz. We attempted to resolve the above problems by selecting JFETs that have large transconductance. low noise characteristic and low power dissipation. and by optimization of the operating condition of the JFETs. 2. I. Experimental
wethod
An 178 pm x 178 pm Nb/Al-AIOz/ Nb STJ (No.16) [5.13] with a low leakage current of 5.8 nA and a capacitance of 1900 pF was used as a standard STJ detector for the optimization of preamplifier. A suitable JFET was selected and optimum operating conditions of the JFETs for the preamplifier were obtained. The JFET. called CA2003 in a Canberra-2003T preamplifier that is often applied to STJs [9], was used as a reference JFET for the selection of JFET and the evaluation of its noise characteristic. Four kinds of JFETs, i.e.. 2SK 147,2SK 190.2SK162 and 2SK291, having transconductances larger than 40 mS, were chosen for this experiment. Table 1 shows the main electronic characteristics of these JFETs. The first three are used for low-noise and low-frequency amplification. The last one for lownoise and high-frequency amplification. At first, we preliminarily investigated the suitable number of JFETs to connect them in parallel for low-noise amplification of STJ signals, taking account of STJ capacitance as large as 1900 pF. It is found that parallel connection of four JFETs is best when the noise dependence of the Canberra-2003T
M. Ukibr et al./Nucl.
Table 1 Main electronic
characteristics
Instr. and Meth. in Phys. Rex A 401 (1997) 399-308
4-JFETs. Signals from the preamplifier were amplified with pulse shaping by a Canberra-2021 spectroscopy amplifier, and the output signals were fed to a multichannel analyzer. The root mean square(RMS) voltage of the noise at the spectroscopy amplifier output was measured by using a RMS voltmeter. The equivalent noise charge (ENC) of the preamplifier was calculated from it. Also. power spectral densities of the noise at the preamplifier output were measured by a spectrum analyzer.
of four kinds of JFETs
Electronic characteristcs
CA2003
2SK190
2SK147
2SK162
2SK291
Transconductance
70”
45
40
45
45
Y, (mS) Gate-source capacitance
50b
75
15
55
C,, (PF) 50” Zero bias drain current IDss (mA)
50
30
30
8.5
50
a Measurement value. b Estimated value.
2.17. Experimental
condition
The STJ was cooled down to 0.4 K by a charcoal absorption 3He cryostat. The first stage of the preamplifier, which consists of the 4-JFETs, a feedback resistance, a feedback capacitance and a test capacitance, was constructed on a Teflon board, and was mounted on a 77 K cooling stage in the cryostat. The temperature of the 4-JFETs was adjusted to around 130 K by changing the thickness of the Teflon board.
preamplifier on the input capacitance is considered. Consequently, the four parallel connection of JFETs (4-JFETs) was used for evaluation of each JFET in the following experiments. An ORTEC120 preamplifier was used in all experiments to adjust the drain current of each JFET. Fig. 1 shows the circuit configuration of the preamplifier with
1 Cooled Area
Bias current
JFET
/
/
_1 47OOOpF
+I? IPF
STJ
i
Fig. 1. Circuit
configuration
301
of the preamplifier
using 4-JFETs
The noise characteristics for each JFET (CA2003, 2SK147, 2SK190, 2SK162 and 2SK291) were measured after the STJ was cooled down to 0.4 K. The drain current of each JFET was changed from 5 to 50 mA and the drain-source voltage was 3 or 5 V. Under these conditions, the RMS voltage of the noise for a shaping time of 0.25-12 ps were measured at different drain currents. The ENC of the preamplifier was calculated from the measured RMS voltage by introduction of a conversion coefficient, that was obtained in calibration using a Si surface barrier detector and an ‘“‘Am alpha source.
Table 2 Optimum
operational
Optimum conditiion
CA2003
Drain current I rImIn(mA) Gate-source voltage I;. (V) Power dissipation
33
for five kinds of JFETs
2SKlYO ‘SK147
5
3.0
30
3.0
100
30
3.0
150
3.0
90
,
,
.
,
,
.
Bias current of STJ Capacitance of STJ
results
The noise characteristics in the case of the CA2003, named as the reference JFET, were measured. Dependencies of the ENC on the shaping time of 0.25-12 ps are shown in Fig. 2. One can see that the best noise performance for the CA2003 is obtained at a drain current of 33mA. The optimum drain currents for all JFETs are shown in Table 2. It is confirmed that each JFET has different optimum drain current. Fig. 3 shows the obtained ENC characteristic for each JFET at the respective optimum drain
2SK2Yl
30
5.0
15
ZSKl67
120
ImW)
10000 .
2.3. Expedtentnl
conditions
- -2SK147 -
01 0
,
.
,
.
5 8nA 19OOpF
(Vlls=5,0V,I,a,n=30mA)
CA2003 (VDs=3 0V,IDca,n=33mA)
1’.“““““’
2
4
6
8
10
12
14
Tsp(blW Fig. 3. Dependence JFETs
of ENC on shaping
time r_, for five kinda of
4.CA2003 JFET preamplifier Draiwsource Voltage Capacitance of STJ
0
2
4
3.OV
: 19OOpF
I
I
I
t
6
8
10
12
3
1
14
$w=C)
Fig. 2. ENC measured a CA’OO?.
as a function
of drain currents(I,,,,,)
for
current. The minimum ENC for the 2SK190 is 1450 RMS electrons, and it is equal to that for the CA2003 at a shaping time of 2 ps. The ENC characteristic for the 2SK190 is almost flat at a shaping time of 1.5-12 vs. It is confirmed that the 2SK190 has very excellent ENC characteristics. On the other hand, the ENC characteristic of the 2SK147 often used for a detector having a large capacitance[ 14, 151 becomes worse especially at a short shaping time less than 2 ps. Fig. 4 shows the dependence of the ENC characteristics for the 2SK190 on the drain current of JFET. From this, it is revealed that the 2SK190 exhibits a good noise performance at low drain current and the best performance is obtained at
,
4000
’
, . ( . , . , . , .
Bias current of STJ : 5.8nA Capacitance of STJ 19OOpF
/a, ~2000
-
E & $000
-
- - ENC from Pulser resolution for CA2003. -
Dralwsource Capacitance 0
2
4
6
Voltage of SlJ 8
10
-
3 OV 19OOpF 12
0
0
+ .‘.‘,‘.‘.‘.I. 2
14
7ENC
from Pulser resolution for 2SK190.
ENC for CA2003 ENC of 2SK190 4
6
8
10
12
14
~sp(~sec)
~sp(l-lSeC)
Fig. 4. ENC measured a ?SKl90.
as a function
of drain currents(l,,,,,)
TOI
a drain current of 5 mA. The total power dissipation in the 4-JFET is only 60 mW. Therefore, the consumption of liquid nitrogen is about l/7 of that of the CA2003. Next, the correlation between the practical energy resolution and the ENC was investigated in case of the CA2003 and the 2SK190. The FWHM energy resolution for pulser signals were measured at a bias current of 5.8 nA for the STJ as a function of shaping time. The results are shown in Fig. 5. The dependencies of the FWHM energy resolution on the shaping time are almost the same as the energy resolution calculated from the ENC. Consequently, it is confirmed that the noise characteristics of the charge-sensitive preamplifier can be evaluated by using the RMS voltage in case of a short shaping time. It can be considered that the disagreement between their characteristics at a shaping time larger than 6 us is due to a baseline drift.
The power spectral densities of the noise in the output of the preamplifier having the above-described 4-JFETs were measured as a function of frequency by a spectrum analyzer because the
Fig. 5. Correlation between ENC a function of shaping time s,,,.
and
Pulser
resolution
as
10-5
IO” Bias current of STJ
5 8ti
lo-'O 01
100 Fkxjuency(ktlZ)
Fig. 6. Power spectral a function of frequency JFETs.
densities of the preamplifier noise as in the 0.1. 100 kHz range for 3 kinds of
dependencies of ENC on the shaping time of the spectroscopy amplifier vary very much with the kind of JFETs. Fig. 6 shows those spectral densities in the case of CA2003,2SK190 and 2SK147 in the range O.lLlOO kHz. One can see that the spectral density for the XK190 is lower than that of the CA2003 in the whole frequency range and especially that the difference between them becomes larger in the range lower than 2 kHz. This result
supports that the ENC characteristic of the 2SK 190 is almost flat at a shaping time from 1.5 to 12 ps. In addition, it is considered that the distortion of ENC in case of the 2SK147 at a shaping time less than 2 ys is due to residual noise components in the range above 10 kHz in the spectral density.
3. Formula for simulation of the noise characteristic If a formula that can simulate the ENC characteristic of the preamplifier is obtained, we easily can optimize the preamplifier for STJs with different junction size, leakage current, dynamic resistance and so on. Therefore, such a formula was developed.
Mathematical formulas for the description of ENC of a charge-sensitive preamplifier have been proposed and evaluated by several groups [ 16-191. These formulas are mainly adopted to detectors having a capacitance less than 100 pF such as semiconductor detectors. In case of the capacitance more than 100 pF, Gramsch et al.[20] have proposed a formula for a Si PIN photodiode that is used for radiation measurements. Also. a qualitative evaluation of the noise of STJ detectors with preamplifiers has been performed by Jochum et al. 11211. We attempted to evaluate the measured ENC characteristics by adopting those formulas with operational parameters of the STJ and the JFET. However, the dependencies of the calculated ENC on the shaping time were different from those of the measured ones. Therefore, a new formula based on those proposed by Radeka and Goulding et al. [16, 171 was developed in order to represent the ENC of the preamplifier for STJs having a large capacitance. 1900 pF, for example. This formula can only be adopted to cooled-JFET preamplifiers. The developed formula was defined as follows:
A, =
((C,,iC,p + (c,&p)
A,, = l,/(r,,2t,
+ 3S,/‘Tp),
12) (3)
where A, is a matching factor k, the Boltzmann’s constant, Tr, T, the temperature of the JFET and detector. C,,. C,, the gate-source capacitance of the JFET and the capacitance of the detector. g,,, the transconductance of the JFET, ?p the peaking time (peaking time of a unipolar pulse shape depends on the pulse shaping systems), 2.35r,, for the Canberra-2021 spectroscopy amplifier [22] in this work. Id the leakage(bias) current of the detector, R, the dynamic resistance of the detector, A,:,-. the l/‘noise coefficient, and A,, .A,,, r,, the amplitude of the generation-recombination noise, the generationrecombination coefficient and the characteristic generation time of the trapping level, respectively. The first term represents the thermal noise in the conducting channel of a JFET and varies depending on the capacitance of the detector and the inversed transconductance. The second term represents the shot noise of the detector and changes with the leakage current of the detector. The third term, the thermal noise of the detector, depends on the inversed dynamic resistance of the detector. The fourth term is a l[f-type noise and varies with the capacitance of the detector. The fifth term. the generation-recombination noise [ 17.233351 in a gate depletion layer of the JFET, changes with the capacitance of the detector. However, the fifth term has usually been neglected except in special cases because this term has only a small contribution to the total noise in case of a capacitance less than 100 pF. Goulding et al. pointed out that the fifth term contributes to the total noise dominantly in case of a cooled-JFET preamplifier using a 2N4416 for a high-resolution Si(Li) X-ray detector[l7,35] because the ENC of the fifth term is larger than that of other terms in the above-described case. Similarly, the fifth term becomes dominant in a detector having a capacitance larger than 100 pF because the term increases with an order of square of the capacitance Cd + C,,. 3.2. Evaluation
of the jknula
The ENCs represented by the new formula compared with the ENCs that were measured
were with
Table 3 Operational
130
conditions
1900
of the preamplifier
15
45
using 2SKl90
0.9
and 178 pm x 178 pm Nb,,AI-AIO,;Nb
5.8
0.4
3.6x 10-l”
50
,
4000, 4.2SK190
JFETpreamplifier
Bias current of STJ I Capacitance
of STJ
4-2SK190
: 5 8nA 19OOpF -
1st term
-6-
2nd term
U
3rd term
--t
4th term
t
5th term
STJ
,
,
1.8x IO-”
1.2~ IO-’
.
.
,
1 ,
(
. ,
JFET preamplkr
81as current of STJ
58nA
Capacitance
19OOpF
of STJ
z3000 E
i 52000
--c-Summation
-
F t?
I51000
-
--ENC
-
0
0
2
4
6 8 ~s,(,wec)
10
12
14
Fig. 7. ENCs of five terms calculated by usmg a single set of parameters in Table 3 and the summation of these terms in Eq.
0
summed ENC summed
“~‘~t~I.‘~‘~ 2
4
from the lstto the 5th term of Eq(1 from the 1st to the 4th term of Eq(1) 6
Fig. 8. Dependencies of calculated on the shaping time r%F.
8
10
12
ENCs and measured
j I4
ENCs
(1).
an optimized preamplifier using 4-JFETs of the 2SK190 type and an 178 urn x 178 urn Nb/ Al-AlO?/Nb STJ having a capacitance of 1900 pF and being operated with a bias current of 5.8 nA. Table 3 shows operational conditions of the preamplifier and the STJ. The values of the five terms numerically calculated by Eq. (1) with the set of parameters in Table 3 and the summation of these terms are plotted in Fig. 7. One can see that the fifth term strongly influences the ENC at a shaping time larger than 3 us. In order to investigate the effect of the fifth term, the ENC summed up from the first to fourth term. and that to the fifth term and the measured ENCs are plotted in Fig. 8. respectively. The measured ENCs can be fitted excellently with the calculated ENCs including the contribution of fifth term. Therefore, it is revealed that the generation-recombination effect in the JFET dominates the ENC and
has great influence on the energy resolution when a cooled-JFETs charge-sensitive preamplifier is used with a detector having a large capacitance. Furthermore, the ENC characteristics were measured at several bias currents of the STJ to evaluate whether this formula can be applicable at different bias currents. Fig. 9 shows the ENC characteristics calculated by the formula and the measured ENC characteristics at bias currents of 1 nA and 12 nA. Both results agreed at the two cases of bias currents. It is confirmed that the new formula can represent the ENC even if the bias current of STJ is changed.
The ENC characteristics of preamplifiers when using the 2SK147 and the 2SK190 as a 4-JFET in the first stage are quite different though operational
306
1
.
Measured
ENC at ld=1n4
-Calculated 0
ENC at Ic=l nA
Measured
800C
ENC at ld=lZrtA
-Calculated
-z
ENC at Id=12nA
s
\
$j6000 -ii 2 .54000 ks W 2000 4m2SK190
JFET
Capacitance 2
preamplkr
of STJ
1900pF
I.
I.
I
I
4
6
8
10
? SP
81.
12
0 14
t 0
2
4
6
IO
12
I 14
rSP(wc)
(u=C)
Fig. 9. Comparison of calculated ENCs and measured bias currents of 1 and 17 nA for the STJ.
8
ENCs at
conditions and electronic characteristics of the two JFETs are almost the same. Therefore, the ENC characteristics of the preamplifier using cooled 4-JFETs with the 2SK147 were analyzed using the formula. It is examined whether the difference between the 2SK147 and the 2SK190 is caused by the fifth term. When parameters for the fifth term. t, = 7.0x lo-’ and A, = 7.0x 10-l’ are used for the calculation, the calculated ENC characteristics agree very well with the measured ENC characteristics. The results are shown in Fig. 10. Since the specific noise frequency for trapping ~~ is shorter and its amplitude A, is larger, the noises produced by the generation-recombination effect in a 2SK 147 shifts to higher frequency and its intensity increases. Also, this result corresponds well with the spectral density of the noise in the output signals of the preamplifier measured by a spectrum analyzer. Next, by using the formula, the optimum number of 2SK190s to use in the preamplifier for the STJ, was obtained. The ENCs were calculated by Eq. (1) with operational parameters used when the number of 2SK190 is one, two, four and eight. These results were shown in Fig. 11. The noise characteristics are improved with an increase in the number of 3SK19O’s. One can see that the ENC characteristics are best when the number of 2SK190s is four.
Fig. IO. Comparison of calculated ENC and measured ENC of a 3-2SK147 preamplifier: (a) solid line for calculated ENC. (b) circles for measured ENC.
Bias cur&
5&IA
of STJ
Capacitance
of STJ
19OOpF
--u
8.2SK190
i 2
4
6
8
10
12
14
?sm(wecI r Fig. I I. Comparison of ENC characteristics ferent number of 3SK190.
calculated
for dif-
On the other hand, the ENC characteristics become worse when the number is eight at a shaping time more than 3 ps. Consequently, we can conclude that the four parallel connection is the most suitable for our STJs.
M. Vkibr et al. iMrc1. It~stv. and Meth. irr Phvs. Rrs. ‘4 1101 (1997) 179%308
2000
I
4-2SK190 JFET preamplifier No.17 STJ Bias current of STJ
,
.
’
Pulser
: 1.44nA
1500 5 G 5ul 1000
FWHM 51eV +
E
I_
z 0
500
-0
1000
500
Channel Fig. 12. Spectrum
measured
1500
2000
No
for 5.9 keV X-ray of ‘ZFe.
307
design of a preamplifier for STJs having different junction sizes, leakage currents, dynamic resistances and so on. It was confirmed that the term due to the generation-recombination noise caused by traps in a gate depletion layer of JFET has a great influence on the energy resolution when a cooled-JFET charge-sensitive preamplifier is used with a detector having a large capacitance. This suggests that we select a JFET with low generation-recombination noise for the cooledJFET preamplifier Consequently, the FWHM energy resolution of 66 eV was obtained for 5.9 keV X-ray by the 178 urn x 178 urn Nb,lAlLAlO,/Nb STJ with a bias current of 1.44nA and the optimized preamplifier. and this is the best among those of Nb-based STJs presented so far.
Acknowledgements 4. Application
of an optimized preamplifier
An 178 urn x 178 urn Nb/Al-AlO,/Nb STJ (No. 17) has the lowest leakage current (1.44 nA) in our Nb-based STJs. Therefore, we made up an X-ray spectrometer which consists of this STJ and the optimized preamplifier. The measured spectrum for 5.9 keV X-ray of 55Fe is shown in Fig. 12. The best FWHM energy resolution of 66 eV for 5.9 keV X-ray and 51 eV for pulser signals was obtained at a shaping time of 3 us. Then. the intrinsic FWHM energy resolution of No. 17 STJ was 42 eV.
5. Conclusion A low-noise charge-sensitive preamplifier with cooled 4-JFETs was developed to improve the energy resolution of a Nb-based superconducting tunnel junction having a large capacitance and a low bias current. A 2SK 190 was selected from five kinds of JFETs as the optimum JFET for the preamplifier of the STJ detector having a large capacitance. The low noise characteristics at a shaping time of 1.5 us to 12 us were obtained by using this JFET. A formula that represents the ENC characteristic for a preamplifier was developed to optimize the
The authors would like to thank T. H. Harano, T. Nakamura for their helpful encouragement and support. A special thanks goes to K. Ara and H. Yamagishi valuable support and comments.
Kakuta. advice. note of for the
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. __ __ fii!B ELSEYIER
Nuclear
A low-noise
Instruments
Secl~on A
and Methods in Physics Research A 401 (1997) 299-308
charge-sensitive preamplifier for an X-ray detector a Nb-based superconducting tunnel junction
M. Ukibe”*b**, M. Kishimoto”,
M. Katagiri”,
M. Kurakadoc,
of
M. Nakazawa”*b
“Advanced Suence Reseawh Center. Japarl Atomic Energ? Research Imtitute, Tokai-mura. Naka-gun. Iharaki-ken. bDepartnlent of Quantum Engirwwing artd ~Istems Science. Utziwmit~, qf Tohyvo, Tobo, Japan ‘Advanced Trchnologv Research Lahomtories, Nippon Steel Corporatiorl, Japan
319-l 1. Japan
Received 26 May 1997; received in revised form 15 July 1997
Abstract A low-noise charge-sensitive preamplifier with cooled 4-JFETs was developed for an X-ray detector of a Nb-based superconducting tunnel junction with a large capacitance and a low leakage current. The low-noise characteristics for a wide range of shaping times were obtained by using a selected JFET, 2SK190 in the first stage of the preamplifier. A new formula to simulate the preamplifier noise was developed and applied to the optimization of preamplifier design. A high-energy resolution of 66 eV for 5.9 keV X-ray and 51 eV for pulser signals was obtained by using the optimized preamplifier with an X-ray detector of a 178 pm x 178 pm Nb/Al-AlO,/Nb superconducting tunnel junction with a junction capacitance of 1900pF and a bias current of 1.44nA. Ke~vmrls:
High-energy
Charge-sensitive resolution
preamplifier;
Cooled
JFET;
X-ray detector;
1. Introduction A radiation tunnel times
junction
detector
higher-energy
a semiconductor
using
(STJ) is expected
a superconducting to have
resolution detector
than
because
has a small energy gap of meV that is much smaller than that ductors. X-ray detectors with STJs therefore, investigated in many groups superconductor
about that
the
40 of
metallic
in the order of semiconhave been, in order to
*Corresponding author. Tel: + 81 29 282 6080; fax: + Sl 29 282 6122: e-mail; [email protected]
0168-9002./97/$17.00 #?I 1997 Elsevier Science B.V. All rights reserved PI1 SO168-9002(97)00967-4
Nb-based:
Superconducting
tunnel junction;
realize X-ray spectroscopy with good energy resolution, but the theoretically expected resolution has not been achieved yet. Hence, the approach to improve the energy resolution of STJ X-ray detectors is still of a challenging one. There are two main approaches to improve the energy resolution of STJ detectors. One is the utilization of trapping effect in an STJ in order to efficiently collect electrons (i.e. quasiparticles) produced by X-ray absorption in a superconductor [l]. By using an Al layer for trapping the quasipartitle. Mears et al. could get electrons in their STJ detector as many as ten times more than by a Nb layer [2]. In this case, the characteristics of the
tunnel junction become generally worse, namely, a low dynamic resistance, a large leakage current and a long charge collection time resulting due to the small energy gap of an Al superconductor as compared with that of a Nb superconductor. Hence, they applied a current-sensitive preamplifier using SQUIDS to deal with electron signals produced in the STJ because the influence of the low dynamic resistance and the large capacitance of the STJ on the noise performance of the SQUID is negligibly small [3,4]. They obtained a FWHM energy resolution of 29 eV for 5.9 keV X-ray by an 100 pm x 100 pm Nb/Al,/AlO,/Al/Nb STJ having 200 nm Al trapping layers on both sides of the tunnel barrier at 0.2 K. Another approach is the improvement of the STJ characteristics by fabricating a perfect STJ structure by which a low leakage current. a small diffusion effect for the quasiparticles produced by X-rays, a constant charge collection time, etc., result [S]. The great advantage of this approach is that X-ray spectroscopy with high counting rate can be realized because the short charge collection time less than several ps is reserved. In this case. a charge-sensitive preamplifier using Junction-type Field Effect Transistors (JFETs) is adopted with this STJ in a similar way as that for a semiconductor detector. Here the optimal design of the preamplifier being adaptive to a large capacitance of the STJ is an essential problem. We attempted to improve the energy resolution of the Nb-based STJ detector having a low leakage current by the latter approach. In this paper, we describe a charge-sensitive preamplifier optimized for the STJ, a formula for simulation of the noise characteristic and X-ray detection with an excellent energy resolution by the optimized preamplifier in the following sections.
2. Experiment
for optimization
of the preamplifier
It is necessary to optimize the charge-sensitive preamplifier for an STJ having a large capacitance in order to develop a high-resolution X-ray detector. It is preferable that the optimized preamplifier has good noise performance at a long shaping time of the spectroscopy amplifier because the
effective quasiparticle lifetime in the STJ becomes longer if the crystallization of the superconductor layer in the STJ is perfect [6--81. Since the capacitance of an STJ is much larger than that of semiconductor detectors, it is necessary to increase the transconductance of the JFET used for the first stage in the preamplifier. Usually. a cooled JFET having a large transconductance has been used to decrease the preamplifier noise resulting from a large capacitance of the STJ [5,9%12]. Also, parallelly connected JFETs have been used for this purpose though they dissipate more power [l 11. On the other hand, an excellent energy resolution is obtained with a long shaping time, when the JFET has low noise characteristics at frequencies less than 10 kHz. We attempted to resolve the above problems by selecting JFETs that have large transconductance. low noise characteristic and low power dissipation. and by optimization of the operating condition of the JFETs. 2. I. Experimental
wethod
An 178 pm x 178 pm Nb/Al-AIOz/ Nb STJ (No.16) [5.13] with a low leakage current of 5.8 nA and a capacitance of 1900 pF was used as a standard STJ detector for the optimization of preamplifier. A suitable JFET was selected and optimum operating conditions of the JFETs for the preamplifier were obtained. The JFET. called CA2003 in a Canberra-2003T preamplifier that is often applied to STJs [9], was used as a reference JFET for the selection of JFET and the evaluation of its noise characteristic. Four kinds of JFETs, i.e.. 2SK 147,2SK 190.2SK162 and 2SK291, having transconductances larger than 40 mS, were chosen for this experiment. Table 1 shows the main electronic characteristics of these JFETs. The first three are used for low-noise and low-frequency amplification. The last one for lownoise and high-frequency amplification. At first, we preliminarily investigated the suitable number of JFETs to connect them in parallel for low-noise amplification of STJ signals, taking account of STJ capacitance as large as 1900 pF. It is found that parallel connection of four JFETs is best when the noise dependence of the Canberra-2003T
M. Ukibr et al./Nucl.
Table 1 Main electronic
characteristics
Instr. and Meth. in Phys. Rex A 401 (1997) 399-308
4-JFETs. Signals from the preamplifier were amplified with pulse shaping by a Canberra-2021 spectroscopy amplifier, and the output signals were fed to a multichannel analyzer. The root mean square(RMS) voltage of the noise at the spectroscopy amplifier output was measured by using a RMS voltmeter. The equivalent noise charge (ENC) of the preamplifier was calculated from it. Also. power spectral densities of the noise at the preamplifier output were measured by a spectrum analyzer.
of four kinds of JFETs
Electronic characteristcs
CA2003
2SK190
2SK147
2SK162
2SK291
Transconductance
70”
45
40
45
45
Y, (mS) Gate-source capacitance
50b
75
15
55
C,, (PF) 50” Zero bias drain current IDss (mA)
50
30
30
8.5
50
a Measurement value. b Estimated value.
2.17. Experimental
condition
The STJ was cooled down to 0.4 K by a charcoal absorption 3He cryostat. The first stage of the preamplifier, which consists of the 4-JFETs, a feedback resistance, a feedback capacitance and a test capacitance, was constructed on a Teflon board, and was mounted on a 77 K cooling stage in the cryostat. The temperature of the 4-JFETs was adjusted to around 130 K by changing the thickness of the Teflon board.
preamplifier on the input capacitance is considered. Consequently, the four parallel connection of JFETs (4-JFETs) was used for evaluation of each JFET in the following experiments. An ORTEC120 preamplifier was used in all experiments to adjust the drain current of each JFET. Fig. 1 shows the circuit configuration of the preamplifier with
1 Cooled Area
Bias current
JFET
/
/
_1 47OOOpF
+I? IPF
STJ
i
Fig. 1. Circuit
configuration
301
of the preamplifier
using 4-JFETs
The noise characteristics for each JFET (CA2003, 2SK147, 2SK190, 2SK162 and 2SK291) were measured after the STJ was cooled down to 0.4 K. The drain current of each JFET was changed from 5 to 50 mA and the drain-source voltage was 3 or 5 V. Under these conditions, the RMS voltage of the noise for a shaping time of 0.25-12 ps were measured at different drain currents. The ENC of the preamplifier was calculated from the measured RMS voltage by introduction of a conversion coefficient, that was obtained in calibration using a Si surface barrier detector and an ‘“‘Am alpha source.
Table 2 Optimum
operational
Optimum conditiion
CA2003
Drain current I rImIn(mA) Gate-source voltage I;. (V) Power dissipation
33
for five kinds of JFETs
2SKlYO ‘SK147
5
3.0
30
3.0
100
30
3.0
150
3.0
90
,
,
.
,
,
.
Bias current of STJ Capacitance of STJ
results
The noise characteristics in the case of the CA2003, named as the reference JFET, were measured. Dependencies of the ENC on the shaping time of 0.25-12 ps are shown in Fig. 2. One can see that the best noise performance for the CA2003 is obtained at a drain current of 33mA. The optimum drain currents for all JFETs are shown in Table 2. It is confirmed that each JFET has different optimum drain current. Fig. 3 shows the obtained ENC characteristic for each JFET at the respective optimum drain
2SK2Yl
30
5.0
15
ZSKl67
120
ImW)
10000 .
2.3. Expedtentnl
conditions
- -2SK147 -
01 0
,
.
,
.
5 8nA 19OOpF
(Vlls=5,0V,I,a,n=30mA)
CA2003 (VDs=3 0V,IDca,n=33mA)
1’.“““““’
2
4
6
8
10
12
14
Tsp(blW Fig. 3. Dependence JFETs
of ENC on shaping
time r_, for five kinda of
4.CA2003 JFET preamplifier Draiwsource Voltage Capacitance of STJ
0
2
4
3.OV
: 19OOpF
I
I
I
t
6
8
10
12
3
1
14
$w=C)
Fig. 2. ENC measured a CA’OO?.
as a function
of drain currents(I,,,,,)
for
current. The minimum ENC for the 2SK190 is 1450 RMS electrons, and it is equal to that for the CA2003 at a shaping time of 2 ps. The ENC characteristic for the 2SK190 is almost flat at a shaping time of 1.5-12 vs. It is confirmed that the 2SK190 has very excellent ENC characteristics. On the other hand, the ENC characteristic of the 2SK147 often used for a detector having a large capacitance[ 14, 151 becomes worse especially at a short shaping time less than 2 ps. Fig. 4 shows the dependence of the ENC characteristics for the 2SK190 on the drain current of JFET. From this, it is revealed that the 2SK190 exhibits a good noise performance at low drain current and the best performance is obtained at
,
4000
’
, . ( . , . , . , .
Bias current of STJ : 5.8nA Capacitance of STJ 19OOpF
/a, ~2000
-
E & $000
-
- - ENC from Pulser resolution for CA2003. -
Dralwsource Capacitance 0
2
4
6
Voltage of SlJ 8
10
-
3 OV 19OOpF 12
0
0
+ .‘.‘,‘.‘.‘.I. 2
14
7ENC
from Pulser resolution for 2SK190.
ENC for CA2003 ENC of 2SK190 4
6
8
10
12
14
~sp(~sec)
~sp(l-lSeC)
Fig. 4. ENC measured a ?SKl90.
as a function
of drain currents(l,,,,,)
TOI
a drain current of 5 mA. The total power dissipation in the 4-JFET is only 60 mW. Therefore, the consumption of liquid nitrogen is about l/7 of that of the CA2003. Next, the correlation between the practical energy resolution and the ENC was investigated in case of the CA2003 and the 2SK190. The FWHM energy resolution for pulser signals were measured at a bias current of 5.8 nA for the STJ as a function of shaping time. The results are shown in Fig. 5. The dependencies of the FWHM energy resolution on the shaping time are almost the same as the energy resolution calculated from the ENC. Consequently, it is confirmed that the noise characteristics of the charge-sensitive preamplifier can be evaluated by using the RMS voltage in case of a short shaping time. It can be considered that the disagreement between their characteristics at a shaping time larger than 6 us is due to a baseline drift.
The power spectral densities of the noise in the output of the preamplifier having the above-described 4-JFETs were measured as a function of frequency by a spectrum analyzer because the
Fig. 5. Correlation between ENC a function of shaping time s,,,.
and
Pulser
resolution
as
10-5
IO” Bias current of STJ
5 8ti
lo-'O 01
100 Fkxjuency(ktlZ)
Fig. 6. Power spectral a function of frequency JFETs.
densities of the preamplifier noise as in the 0.1. 100 kHz range for 3 kinds of
dependencies of ENC on the shaping time of the spectroscopy amplifier vary very much with the kind of JFETs. Fig. 6 shows those spectral densities in the case of CA2003,2SK190 and 2SK147 in the range O.lLlOO kHz. One can see that the spectral density for the XK190 is lower than that of the CA2003 in the whole frequency range and especially that the difference between them becomes larger in the range lower than 2 kHz. This result
supports that the ENC characteristic of the 2SK 190 is almost flat at a shaping time from 1.5 to 12 ps. In addition, it is considered that the distortion of ENC in case of the 2SK147 at a shaping time less than 2 ys is due to residual noise components in the range above 10 kHz in the spectral density.
3. Formula for simulation of the noise characteristic If a formula that can simulate the ENC characteristic of the preamplifier is obtained, we easily can optimize the preamplifier for STJs with different junction size, leakage current, dynamic resistance and so on. Therefore, such a formula was developed.
Mathematical formulas for the description of ENC of a charge-sensitive preamplifier have been proposed and evaluated by several groups [ 16-191. These formulas are mainly adopted to detectors having a capacitance less than 100 pF such as semiconductor detectors. In case of the capacitance more than 100 pF, Gramsch et al.[20] have proposed a formula for a Si PIN photodiode that is used for radiation measurements. Also. a qualitative evaluation of the noise of STJ detectors with preamplifiers has been performed by Jochum et al. 11211. We attempted to evaluate the measured ENC characteristics by adopting those formulas with operational parameters of the STJ and the JFET. However, the dependencies of the calculated ENC on the shaping time were different from those of the measured ones. Therefore, a new formula based on those proposed by Radeka and Goulding et al. [16, 171 was developed in order to represent the ENC of the preamplifier for STJs having a large capacitance. 1900 pF, for example. This formula can only be adopted to cooled-JFET preamplifiers. The developed formula was defined as follows:
A, =
((C,,iC,p + (c,&p)
A,, = l,/(r,,2t,
+ 3S,/‘Tp),
12) (3)
where A, is a matching factor k, the Boltzmann’s constant, Tr, T, the temperature of the JFET and detector. C,,. C,, the gate-source capacitance of the JFET and the capacitance of the detector. g,,, the transconductance of the JFET, ?p the peaking time (peaking time of a unipolar pulse shape depends on the pulse shaping systems), 2.35r,, for the Canberra-2021 spectroscopy amplifier [22] in this work. Id the leakage(bias) current of the detector, R, the dynamic resistance of the detector, A,:,-. the l/‘noise coefficient, and A,, .A,,, r,, the amplitude of the generation-recombination noise, the generationrecombination coefficient and the characteristic generation time of the trapping level, respectively. The first term represents the thermal noise in the conducting channel of a JFET and varies depending on the capacitance of the detector and the inversed transconductance. The second term represents the shot noise of the detector and changes with the leakage current of the detector. The third term, the thermal noise of the detector, depends on the inversed dynamic resistance of the detector. The fourth term is a l[f-type noise and varies with the capacitance of the detector. The fifth term. the generation-recombination noise [ 17.233351 in a gate depletion layer of the JFET, changes with the capacitance of the detector. However, the fifth term has usually been neglected except in special cases because this term has only a small contribution to the total noise in case of a capacitance less than 100 pF. Goulding et al. pointed out that the fifth term contributes to the total noise dominantly in case of a cooled-JFET preamplifier using a 2N4416 for a high-resolution Si(Li) X-ray detector[l7,35] because the ENC of the fifth term is larger than that of other terms in the above-described case. Similarly, the fifth term becomes dominant in a detector having a capacitance larger than 100 pF because the term increases with an order of square of the capacitance Cd + C,,. 3.2. Evaluation
of the jknula
The ENCs represented by the new formula compared with the ENCs that were measured
were with
Table 3 Operational
130
conditions
1900
of the preamplifier
15
45
using 2SKl90
0.9
and 178 pm x 178 pm Nb,,AI-AIO,;Nb
5.8
0.4
3.6x 10-l”
50
,
4000, 4.2SK190
JFETpreamplifier
Bias current of STJ I Capacitance
of STJ
4-2SK190
: 5 8nA 19OOpF -
1st term
-6-
2nd term
U
3rd term
--t
4th term
t
5th term
STJ
,
,
1.8x IO-”
1.2~ IO-’
.
.
,
1 ,
(
. ,
JFET preamplkr
81as current of STJ
58nA
Capacitance
19OOpF
of STJ
z3000 E
i 52000
--c-Summation
-
F t?
I51000
-
--ENC
-
0
0
2
4
6 8 ~s,(,wec)
10
12
14
Fig. 7. ENCs of five terms calculated by usmg a single set of parameters in Table 3 and the summation of these terms in Eq.
0
summed ENC summed
“~‘~t~I.‘~‘~ 2
4
from the lstto the 5th term of Eq(1 from the 1st to the 4th term of Eq(1) 6
Fig. 8. Dependencies of calculated on the shaping time r%F.
8
10
12
ENCs and measured
j I4
ENCs
(1).
an optimized preamplifier using 4-JFETs of the 2SK190 type and an 178 urn x 178 urn Nb/ Al-AlO?/Nb STJ having a capacitance of 1900 pF and being operated with a bias current of 5.8 nA. Table 3 shows operational conditions of the preamplifier and the STJ. The values of the five terms numerically calculated by Eq. (1) with the set of parameters in Table 3 and the summation of these terms are plotted in Fig. 7. One can see that the fifth term strongly influences the ENC at a shaping time larger than 3 us. In order to investigate the effect of the fifth term, the ENC summed up from the first to fourth term. and that to the fifth term and the measured ENCs are plotted in Fig. 8. respectively. The measured ENCs can be fitted excellently with the calculated ENCs including the contribution of fifth term. Therefore, it is revealed that the generation-recombination effect in the JFET dominates the ENC and
has great influence on the energy resolution when a cooled-JFETs charge-sensitive preamplifier is used with a detector having a large capacitance. Furthermore, the ENC characteristics were measured at several bias currents of the STJ to evaluate whether this formula can be applicable at different bias currents. Fig. 9 shows the ENC characteristics calculated by the formula and the measured ENC characteristics at bias currents of 1 nA and 12 nA. Both results agreed at the two cases of bias currents. It is confirmed that the new formula can represent the ENC even if the bias current of STJ is changed.
The ENC characteristics of preamplifiers when using the 2SK147 and the 2SK190 as a 4-JFET in the first stage are quite different though operational
306
1
.
Measured
ENC at ld=1n4
-Calculated 0
ENC at Ic=l nA
Measured
800C
ENC at ld=lZrtA
-Calculated
-z
ENC at Id=12nA
s
\
$j6000 -ii 2 .54000 ks W 2000 4m2SK190
JFET
Capacitance 2
preamplkr
of STJ
1900pF
I.
I.
I
I
4
6
8
10
? SP
81.
12
0 14
t 0
2
4
6
IO
12
I 14
rSP(wc)
(u=C)
Fig. 9. Comparison of calculated ENCs and measured bias currents of 1 and 17 nA for the STJ.
8
ENCs at
conditions and electronic characteristics of the two JFETs are almost the same. Therefore, the ENC characteristics of the preamplifier using cooled 4-JFETs with the 2SK147 were analyzed using the formula. It is examined whether the difference between the 2SK147 and the 2SK190 is caused by the fifth term. When parameters for the fifth term. t, = 7.0x lo-’ and A, = 7.0x 10-l’ are used for the calculation, the calculated ENC characteristics agree very well with the measured ENC characteristics. The results are shown in Fig. 10. Since the specific noise frequency for trapping ~~ is shorter and its amplitude A, is larger, the noises produced by the generation-recombination effect in a 2SK 147 shifts to higher frequency and its intensity increases. Also, this result corresponds well with the spectral density of the noise in the output signals of the preamplifier measured by a spectrum analyzer. Next, by using the formula, the optimum number of 2SK190s to use in the preamplifier for the STJ, was obtained. The ENCs were calculated by Eq. (1) with operational parameters used when the number of 2SK190 is one, two, four and eight. These results were shown in Fig. 11. The noise characteristics are improved with an increase in the number of 3SK19O’s. One can see that the ENC characteristics are best when the number of 2SK190s is four.
Fig. IO. Comparison of calculated ENC and measured ENC of a 3-2SK147 preamplifier: (a) solid line for calculated ENC. (b) circles for measured ENC.
Bias cur&
5&IA
of STJ
Capacitance
of STJ
19OOpF
--u
8.2SK190
i 2
4
6
8
10
12
14
?sm(wecI r Fig. I I. Comparison of ENC characteristics ferent number of 3SK190.
calculated
for dif-
On the other hand, the ENC characteristics become worse when the number is eight at a shaping time more than 3 ps. Consequently, we can conclude that the four parallel connection is the most suitable for our STJs.
M. Vkibr et al. iMrc1. It~stv. and Meth. irr Phvs. Rrs. ‘4 1101 (1997) 179%308
2000
I
4-2SK190 JFET preamplifier No.17 STJ Bias current of STJ
,
.
’
Pulser
: 1.44nA
1500 5 G 5ul 1000
FWHM 51eV +
E
I_
z 0
500
-0
1000
500
Channel Fig. 12. Spectrum
measured
1500
2000
No
for 5.9 keV X-ray of ‘ZFe.
307
design of a preamplifier for STJs having different junction sizes, leakage currents, dynamic resistances and so on. It was confirmed that the term due to the generation-recombination noise caused by traps in a gate depletion layer of JFET has a great influence on the energy resolution when a cooled-JFET charge-sensitive preamplifier is used with a detector having a large capacitance. This suggests that we select a JFET with low generation-recombination noise for the cooledJFET preamplifier Consequently, the FWHM energy resolution of 66 eV was obtained for 5.9 keV X-ray by the 178 urn x 178 urn Nb,lAlLAlO,/Nb STJ with a bias current of 1.44nA and the optimized preamplifier. and this is the best among those of Nb-based STJs presented so far.
Acknowledgements 4. Application
of an optimized preamplifier
An 178 urn x 178 urn Nb/Al-AlO,/Nb STJ (No. 17) has the lowest leakage current (1.44 nA) in our Nb-based STJs. Therefore, we made up an X-ray spectrometer which consists of this STJ and the optimized preamplifier. The measured spectrum for 5.9 keV X-ray of 55Fe is shown in Fig. 12. The best FWHM energy resolution of 66 eV for 5.9 keV X-ray and 51 eV for pulser signals was obtained at a shaping time of 3 us. Then. the intrinsic FWHM energy resolution of No. 17 STJ was 42 eV.
5. Conclusion A low-noise charge-sensitive preamplifier with cooled 4-JFETs was developed to improve the energy resolution of a Nb-based superconducting tunnel junction having a large capacitance and a low bias current. A 2SK 190 was selected from five kinds of JFETs as the optimum JFET for the preamplifier of the STJ detector having a large capacitance. The low noise characteristics at a shaping time of 1.5 us to 12 us were obtained by using this JFET. A formula that represents the ENC characteristic for a preamplifier was developed to optimize the
The authors would like to thank T. H. Harano, T. Nakamura for their helpful encouragement and support. A special thanks goes to K. Ara and H. Yamagishi valuable support and comments.
Kakuta. advice. note of for the
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