Nuclear Instruments and Methods 204 (1983) 479-483 479 North-Holland Publishing Company

A S I M P L E L O W - N O I S E C R Y O G E N I C P R E A M P L I F I E R F O R S I L I C O N S U R F A C E B A R R I E R D E T E C T O R S

M, G E R E T S C H L A G E R

Johannes - Kepler- Universiti~t Linz, Inst i tut f f ir Experimentalphysik , A - 4040 Linz, Austr ia

Received 29 June 1982

A simple high resolution charge sensitive preamplifier for surface barrier detectors (SBD) is described which uses drain feedback for charge restoration. It is shown that electronic noise can be reduced from 5 keV for a standard SBD-preamplifier system to 0.5 keV if one cools the SBD to 77 K and the field effect transistor (FET) to about 100 K and if one uses a SBD-FET assembly which is free from microphonics. The performance is demonstrated by measuring proton induced X-rays of Mg which have an energy of 1.25 keV. This large noise reduction results also in an enhancement of timing resolut ion by a factor 3.

1. Introduction

Silicon surface barrier detectors (SBDs) have been used extensively to measure the energy distribution and time distribution of particles. Usually, commercially available SBD-preampl i f ie r systems are used at 300 K and have a noise width larger than 4 keV. Therefore, particle or photon energies below 10 keV cannot be resolved from the noise distribution. However, cooling the SBD [1-3] as well as the input stage of the pre- amplifier [4-6] will reduce the noise appreciably. Using SBDs for detecting X-rays with energies 1.25 keV ~< E x ~< 8 keV would be very advantageous since it is possible to determine the photopeak efficiency of a standard SBD with an accuracy of better than 1% in that energy range [7]. Our aim is to show the mechanical and electronic construction of a cryogenic SBD-preampl i - tier system capable of measuring X-rays with energies as low as 1.25 keV, which corresponds to K X-rays of magnesium.

2. Preamplifier

Preamplifiers for high resolution cryogenic semicon- ductor detectors normally use a charge sensitive config- uration. To ensure stability of the integrator, charge restoration has to be provided for the feedback capaci- tor. The simplest approach to charge restoration is a resistance larger than 1 GI2 connected in parallel to the feedback capacitor, but this will add noise due to in- creased stray capacitance of the input of the amplifier. Optoelectronic feedback •8,9] eliminates the problems of large resistors, but requires removal of the case of the field effect transistor (FET) and elaborated light shield-

0167-5087/83/0000-0000/$03.00 © 1983 North-Hol land

ing and light attenuation. However, it has been shown [10] that the FET leakage current depends strongly on the drain gate voltage and that one can consequently ensure stability of the charge sensitive preamplifier by drain feedback [10].

Fig. 1 shows the simple electronic circuit diagram of the charge sensitive preamplifier in drain feedback con- figuration, and fig. 2 shows the mechanical arrange- ment. The FET (TI), which is mounted within the vacuum system of the cold finger immediately at the SBD, operates in a common source circuit. The next stage of the amplifier is a bipolar transistor (T2) in a common base circuit. The load of T2 is a constant current source (T3). Provided there is no additional load for T2, the overall voltage amplification will be ap- proximately v = S / 3 3 ( r c E 2 II rCE3) , where S is the trans- conductance of the FET,/33 is the current amplification of T3 and (rcm II FCE3) is the resistance which results if the collector emitter resistances of T2 and T3 are con- nected in parallel. Since the input impedance of the unity gain buffer amplifier (BB3553) is 10 tl I2 we have an open loop voltage amplification v --- 105 at low fre- quencies, which ensures good linearity. No phase com- pensation for stability of the closed loop is necessary. Of course it is very important to minimize the stray capacitance at the input of the buffer amplifier in order to have a large ga in-bandwidth product and high slew- ing rate. The low noise operational amplifiers O1 and 0 2 form together a non-inverting integrator which con- trois the base voltage of T2 and hence the drain voltage of the FET. The feedback capacity C F and the coupling capacity to the pulser Cp are etched onto a double- layered printed circuit. Their values are about 0.3 pF each. The transistor T4 is mounted in thermal contact to the FET-case and was used to measure the case

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temperature of the FET by measuring the forward-bi- ased base emitter voltage. The reverse biased base emitter junction was used to efficiently control the FET-case temperature.

Since the drain feedback configuration is stable only if the detector bias and the preamplifier bias are applied

simultaneously, we also provided the possibility to use resistive feedback which is obtained if the jumpers J2 and J3 are closed and J1 is removed. In the resistive feedback mode the preamplifier is stable without a SBD at its input and therefore this mode is very convenient for preamplifier tests.

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Fig. 2. Mechanical SBD-FET assembly. The entire arrangement shown is in vacuum.

M. Geretschli~ger / Cryogenic preamplifier 481

The noise performance of the preamplifier is mainly determined by the noise contribution of the FET which depends at fixed case temperature on drain voltage and on drain current [10]. However, since the gate current, which is used for charge restoration in drain feedback, depends on drain voltage and drain current too, these two parameters cannot in general be chosen indepen- dently from the actually used SBD.

To test our design we built three prototype SBD-preampl i f ier units. The following procedure was used to obtain minimum noise: At first we selected " g o o d " FETs among 26 specimens of type 2N4416A, using resistive feedback without SBD at the preampli- fier input. Drain voltage and drain current were 6 V and 11 re.A, respectively, and for convenience we did not cool the FETs. We found eight "good" FETs having a noise voltage three times smaller than the worst FET.

Then we determined the opt imum FET-case temper- ature for the three units by changing the thermal resis- tance between cold finger and FET (see fig. 2) a n d / o r by heating the FET case. For this procedure we used drain feedback and a drain resistance R o = 1.5 kI2. The temperature of the SBDs was generally as close as possible to 77 K and their bias voltage was V B = - 1 5 0 V. After that we found the individual opt imum drain resistance value R D for each of our three units which turned out to be 560 $2, 820 I2 and 1.2 k$2, respectively. Finally we tried to adjust the thermal resis- tance between cold finger and FET-case so that we had opt imum FET temperature without heating. These opti- mum temperatures were 92 K, 104 K and 112 K, re- spectively.

3. Mechanical mounting of SBD and FET

Proper mounting of the S B D - F E T unit within the liquid nitrogen cold finger is very important for opti- mum resolution. Large stray capacitance and micro- phonics can greatly increase the noise of the system. We used standard Ortec partially depleted surface barrier detectors and found that they could be cooled without damage to 77 K, although they are not guaranteed below - 3 0 ° C . However, it turned out that "B-mount" SBDs could not be used since the contact spring [11] together with the boiling nitrogen or any other mecha- nic vibration in the room produced microphonic noise equivalent to about 2 keV. For standard SBD-pre - amplifier systems at room temperature this noise is hidden in the electronic noise. To get rid of this micro- phonics we disassembled the B-mount SBDs and used the Si-crystal only (this corresponds [11] to "A-mount") .

Fig. 2 shows the S B D - F E T assembly. The front contact of the SBD is in good thermal contact with the cooled bronze block. Electrically the SBD is insulated from the block by means of a mica disc. Negative bias is

applied to the front contact through a microdot cou- pling. The FET is soldered onto the printed circuit disc which serves as a spring contact between FET gate and SBD and which also contains C F and C v. A plexiglass plate holds the S B D - F E T assembly together. The FET case is cooled via a copper braid. The physical dimen- sions of this braid were adjusted in order to obtain opt imum FET temperature.

In the S B D - F E T arrangement described, the cou- pling capacity between bias connector and gate of the F E T - and hence microphonics - are minimized. Even so, some low frequency microphonics was still barely detectable, originating from small vibrations of the con- nector cables between S B D - F E T assembly and vacuum feedthrough, which were excited by oscillations of the inner tube against the outer tube of our cold fingers. Four small pieces of viton pressed between inner and outer tube eliminated this microphonics completely.

4. Results

To determine energy and time resolution of our three SBD-preampl i f ier units we performed measurements using standard ortec electronics. The energy resolution was measured using MnK~B X-rays of a SSFe radioac- tive source. We used a 716 A shaping amplifier with ~-= 10 bts, a 459 detector bias supply and a 419 preci- sion pulser. The resolutions obtained are given in table 1.

Fig. 3 shows the X-ray spectrum of the 55 Fe radioac- tive source taken with the unit no. 2. The Si-escape peak is clearly resolved from the MnK~a full energy peak. The hump at the high energy side of the full energy peak indicates the MnK~ line which is separated from the MnK~ line by 596 eV.

To demonstrate that SBDs can be used as highly efficient detectors for detecting low energy X-rays we show in figs. 4 and 5 the X-ray spectra of Mg and A1 which correspond to X-ray energies of 1.25 keV and 1.49 keV, respectively. These spectra were taken using system 2. The targets were infinitely thick; 600 keV protons were used to produce the X-rays. There was no

Table 1 Energy resolution (full width at half-maximum) of three SBD-FET systems for MNK,,t~ X-rays (AEx) and an elec- tronic pulser (AEel). The SBD of system no. 2 has a gold front contact of 20/~g/cm 2 which is half the standard thickness.

System no. Detector model no. AE x AEel (eV) (eV)

1 BA-035-025-100 672 643 2 BA-014-025-500-S 607 562 3 BA-014-025 -500 530 506

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absorber between target and SBD. Backscattered pro- tons were prevented from reaching the SBD by means of a small permanent magnet. Only a very small number (not visible in the figures) of backscattered neutral hydrogen atoms reached the SBD.

The question arises whether protons could be mea- sured with a similar energy resolution. However, energy loss straggling within the front contact and the dead layer of the SBD degrades energy resolution. Using the SBD with a gold front contact of 2 0 / x g / c m 2 (system no. 2) we obtained for protons with energies 80 keV ~< Ep ~< 600 keV a full width at half-maximum of 2.7 keY/> Ep >/2.5 keV.

The relatively small charge collection times in a SBD allow it to be used in fast timing experiments and therefore the time resolution of a detector-preamplif ier system is also of importance. We performed a simulated timing test to compare our preamplifier to a 142 A preamplifier. Fig. 6 shows the electronic setup used. The output of an electronic pulser is fed simultaneously to the input of a delay-line amplifier and the test input of a preamplifier (either 142 A or our system no. 1). Using a second delay-line amplifier and two timing single channel analyzers we derived the start and stop signals for a biased time-to-pulse-height converter. Since elec- tronic noise and hence timing resolution depends on

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detector capacity we used similar SBDs (model no. BA-014-025-100, connected by a Microdot-BNC adapter to the 142 A preamplifier and our system no. 1). Fig. 7 shows the time resolution of both systems as a function of equivalent energy which was calibrated using back- scattered protons of known energy. It can be seen that reduction of electronic noise considerably improves the

time resolution too.

References

[1] E. Saki, H.L. Maim and I.L. Fowler, Semiconductor nuclear particle detector and circuits, Nat. Acad. Sci. Publ. 1953, eds. W.L. Brown, W.A. Higinbothan, G.L. Miller and R.L. Chase (National Academy of Sciences, 1969) and Chalk River Report AECL-2762 (1967).

[2] M. Martini, T.A. McMath and I.L Fowler, IEEE Trans. Nucl. Sci. NS-17 (1970) 139.

[3] F. Calligaris, P. Cinti, U. Gabrielli and R. Giaconnich, Nucl. Instr. and Meth. 112 (1973) 591.

[4] T.V. Blalock, IEEE Trans. Nucl. Sci. NS-I1 (1964) 365. [5] J. Llacer and D.F. Maier, IEEE Trans. Nucl. Sci. NS-24

(1977) 317. [6] F.S. Goulding, Nucl. Instr. and Meth. 142 (1977) 213. [7] M. Geretschl~iger, Nucl. Instr. and Meth. 192 (1982) 117. [8] F.S. Goulding, J.T. Walton and R.H. Pehl, IEEE Trans.

Nucl. Sci. NS-17 (1970) 218. [9] D.A. Landis, F.S. Goulding, R.H. Pehl and J.T. Walton,

[EEE Trans. Nucl. Sci. NS-18 (1971) 115. [10] E. Elad, IEEE Trans. Nucl. Sci. NS-19 (1972) 403. [11] EG&G Ortec, Silicon charged particle radiation detectors

instruction manual, p. 2.

Fig. 7. Time resolution of a 142 A preamplifier and of our drain feedback preamplifier as a function of equivalent proton en- ergy.