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Gd microstrip as linear position-sensitive detector for thermal neutrons
Nuclear Instruments and Methods in Physics Research A 424 (1999) 523—532
Space resolution of a Si/Gd microstrip as linear position-sensitive detector for thermal neutrons C. Petrillo *, F. Sacchetti , G. Maehlum , M. Mancinelli Istituto Nazionale per la Fisica della Materia, Unita& di Perugia, I-06123 Perugia, Italy Dipartimento di Fisica, Universita& di Perugia, Via A. Pascoli, I-06123 Perugia, Italy Received 10 August 1998; received in revised form 6 October 1998
Abstract A Si microstrip detector coupled to a Gd converter has been designed for operation as linear position-sensitive detector of thermal neutrons. Test measurements have been carried out on a first prototype. The measured space resolution is reported and discussed in comparison with the results of a Monte Carlo simulation. The space resolution of the Si microstrip detector, given by the response of the sensor to an ideal narrow incoming beam, has been also modeled by the Monte Carlo simulation program. 1999 Elsevier Science B.V. All rights reserved. PACS: 61.12.-q; 29.40.Wk; 29.40.Gx Keywords: Neutron scattering; Solid state detectors
1. Introduction The intensity gain expected from the high fluxes available at next generation neutron sources will result in higher data rates and will be exploited in higher-resolution experiments or smaller sample investigations [1]. The time structure of the neutron beam in a pulsed source, together with the effective intensity gain, will demand an improvement in detector capabilities like the maximum instantaneous rate of data acquisition and/or the space resolution in position sensitive devices.
* Correspondence address: Dipartimento di Fisica, Universita’ di Perugia, Via A. Pascoli, 06123 Perugia, Italy. Tel.: (39) 75 585 3021; e-mail: [email protected].
Moreover, improved performances in both shape flexibility and total coverage surface will be relevant for fully exploiting the neutron source possibilities by new spectrometer designs. Current capabilities and possible improvements of traditional gas detectors for thermal neutrons have been reviewed in Ref. [2] where the limitations on position resolution and maximum data rate achievable in gas detectors are discussed. The best position resolution values reported for the Multi-Wire-Proportional-Chamber (MWPC) detector range from 1.3 to 2 mm Full-Width at HalfMaximum (FWHM) in both dimensions on a 500;500 mm detector [2,3], whereas data rates exceeding 10 s\ mm\ are expected by use of Micro Strip Gas Chambers (MSGC) [4]. In current operation, proportional gas counters yield typical
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values of a few millimeters in position resolution at usable data rates ranging from 10 to 10 s\ over the total area. Approximately the same performances are offered by scintillator detectors. The characteristics of ZnS(Ag)/Li-based detectors, routinely operated on many spectrometers at ISIS, are reviewed in Ref. [5]: a position resolution as good as 3;3 mm at usable data rates of &4;10 over the pixel area is reported for the 4096-pixel SXD detector. Higher resolutions are obtained by image plates [6] or neutron-sensitive Charge Coupled Device (CCD), typically 100 lm. These detectors, however, cannot be employed as realtime counters and their use is confined to specific applications at steady sources and it is ruled out at pulsed sources when the time-of-flight techniques are exploited. Silicon strip detectors are known for their unrivalled space resolution at high counting rates and they are widely employed as particle trackers in high-energy physics. The performances achievable by Si devices in thermal neutron detection have been recently investigated [7—9] with focus on neutron detection efficiency, effective dead time, c-ray rejection and stability. Measurements were carried out on simple detector prototypes whose design was optimized to fit with the parameters to be tested. Neutron data rates up to 5;10 s\ mm\ were found to be accessible [7] with the prototype detectors coupled to VLSI (Very Large Scale Integration) chips [10,11] as multi-channel charge-sensitive preamplifiers, while only preliminary space resolution measurements were carried out [7]. In the present paper, the space resolution of a microstrip detector for thermal neutrons is investigated. The prototype was designed as a small basic unit of a modular expandable linear Position-Sensitive Detector (PSD) for coverage of larger areas. For the Si detector to be a really useful neutron PSD, relatively large areas need to be covered and this can be achieved by the optimized layout of the maximum size available sensor unit. Moreover, even for the single unit, the parallel readout of the rather large number of output channels requires a proper fanout of the signal from the VLSI frontend electronics to the conventional electronics. The space resolution measurements on the
prototype detector are reported and compared with the results of a Monte Carlo simulation.
2. Detector prototype The silicon sensor was a double-sided microstrip with 50 lm pitch and 20;20 mm size operated as a single-sided microstrip for being a linear position sensitive detector. Signals from the front side (p-side) strips were collected while the backplane (n-side) strips were electrically connected together to give a common signal through a resistive load. Considering that not extremely high resolutions are required for normal operation as neutron detector, groups of strips were connected together via ultrasonics microbonding. Each group was coupled to a channel of the readout chip by integrated capacitors having 16.7;6.6 mm size and 100 lm pitch. In Fig. 1a the pattern of the connections is schematically shown. On each side of the sensor, alternate strips are connected to the capacitors thus matching the 100 lm pitch of the capacitor itself. Grouping of four and two alternate strips is then obtained through a common bonding on the same bonding pad. The signal from the group is addressed to one channel of the readout chip. By this configuration, an effective 400 lm pitch sensor, with the central portion at 200 lm pitch, was obtained. The possibility of varying the pitch between 200 and 400 lm on the same sensor was preferred in this prototype version to test the performances obtainable with different configurations in terms of efficiency and noise. Of course, grouping of strips increases the sensor capacitance and, as a consequence, the effective input noise. Nonetheless, the very low-noise preamplifiers employed in the present prototype allowed for safe operation of the sensor with strips connected in groups of four elements. The readout chip, 1;3 mm size, was the 8-channel low-noise charge-sensitive preamplifier and shaper VA (Ide As, Norway). For the detector to be space expandable, the VA chips were mounted on both sides of the sensor. With the strips running vertically, several of these detector units, or similarly designed systems, could be put close together minimising the hindrance of the frontend
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Fig. 1. Schematic of the microstrip detector: (a) strip and connection pattern; (b) detector board layout.
electronics. The first prototype was assembled using 8 VA chips (64 output channels) in order to independently read out the 200 lm pitch central portion of the sensor on both sides. The resulting layout is schematically shown in Fig. 1b. Specifically, the upper (16) and bottom (16) groups of strips (400 lm pitch) were connected to the VAs on the opposite sides of the sensor with the central portion of the sensor (200 lm pitch) independently read out on each side (32 groups, 16 groups per side). The final detector will have a similar design but with constant pitch and one output channel per strip group. The neutron converter was the already tested natural gadolinium which was mounted in contact with the backplane of the sensor, ensuring at the same time the electrical connection of all the strips on the ohmic n-side. Use of the detector in reflection geometry with respect to the neutron beam does not limit the maximum thickness of the Gd converter. A commercial Gd plate, 250 lm thick-
ness, 25;25 mm size was used (Goodfellow, UK). The less energetic line of the conversion electrons following the neutron capture in Gd is observed at &70 keV giving rise to the production of &20000e\, i.e. &3.2 fC released charge inside the 300 lm thick Si sensor. The VA chip has a gain of &28 mV/fC and was set for a peaking time of 1 ls. This chip can only drive a high impedance load, therefore, to increase the gain and to buffer the chip signal, a further amplification stage based on conventional operational amplifiers (quad-amplifiers OP467) was assembled. The 64 output signals from the VA chips were connected through a short flat cable to the amplification board. Both the board allocating sensor and frontend electronics and the auxiliary amplification board were contained inside the same aluminum box. Bias voltage to the sensor and power to the chips were supplied by an external power supply. The 64 amplified signals could be sent to a 64-channels parallel data acquisition
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electronics (like the standard DAE at ISIS [12]). However, for the present test measurements, the 64 signals were separately sent to a PC equipped with a Scope card (ADC, 100 MHz, 8-Bit; Matec Instruments, USA) and a PHA (Pulse Height Analyzer) card (ADC, 8000 channels; Silena, Italy) for collection of single pulses or the full PHA spectrum from the strips. The operation of the detector was tested by using both a b-emitter source (Sr) and monochromatic X-rays from Ag K . a 3. Neutron measurements The prototype detector was tested on the thermal neutron beam of the TRIGA reactor (ENEA-CRE Casaccia, Rome, Italy). Two different experimental setups, schematically shown in Fig. 2a and b, were employed, the main difference being the amount of
c-rays in the incoming beam. No neutron or c-ray shielding was employed in all the experimental tests. In the experimental configuration of Fig. 2a, the white moderated neutron beam emerging from the reactor was monochromatized through the (0 0 2) Bragg reflection in pyrolitic graphite, resulting in the incoming wavelength j"1.42 As . A vertical Soller collimator, 350 mm length and 3 mm slit separation, giving an angular divergence of 0.5°, was inserted after the monochromator. The size of the exit hole was defined by an adjustable cadmium double-slit arranged to 0.31 mm width and 15 mm length. The detector was mounted on a translation stage, 50 lm minimum step, at 28 mm distance from the exit hole with the strips running parallel to the slit length. In such a configuration the incoming beam was rather intense but strongly contaminated by medium energy (1—2 MeV) c-rays, those present in the neutron beam before the
Fig. 2. Schematic of the experimental setup. (a) Detector on the direct beam and mounted on a translation stage. (b) Detector on the double-crystal beam and maintained in a fixed position. S1, S2: Cd slits, 1.2 mm width, 20 mm length. S3: Cd slit, 0.43 mm width, 20 mm length. The slit S3 was mounted on a translation stage.
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monochromator as well as those produced by neutron absorption in the Cd slit. The c-ray beam had therefore a structure broader than that of the neutron beam. The presence of this remarkable c-ray field was exploited to investigate the effect produced on the performance of the detector. The neutron measurements were carried out by translating the detector in steps of 0.1 mm in front of the incoming beam and collecting the PHA spectra at each translation position. The procedure was repeated for various strips. The reason for collecting the full PHA spectra is that the energy dependence of the space resolution can be checked together with the contribution brought about by c-rays having an energy in excess of &250 keV, that is above the maximum energy of conversion electrons. An overall view of the space resolution dependences is presented in Fig. 3 where the PHA spectra collected for one strip are shown as function of both the energy and the translation position. The data in Fig. 3 were normalized to the same value of the intensity integral over the translation position at each energy channel. The normalization was applied in order to enhance the change of the peak width. From Fig. 3, the broadening of the curves versus the position, with increasing the electron energy, is apparent. Therefore, the PHA spectra, measured at each translation position, were integrated over three different energy regions, namely from 10 to 70 keV, from 70 to 170 keV and from 170 to 1000 keV. In Fig. 4a—c the data obtained after
Fig. 3. PHA spectra versus energy and translation position for a reference strip from the 400 lm pitch portion of the sensor. Data collected in the experimental configuration of Fig. 2a.
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integration over these three regions are shown as a function of the position for the same reference strip. The increase in width with increasing the energy is apparent from these figures. Finally, in Fig. 5 the spectra integrated from 10 to 70 keV of two alternate strips belonging to the 200 lm pitch portion of the sensor are shown versus the translation position. As expected, the distance between the peak positions is 400 lm and this figure is representative of the effect expected from a simultaneous readout of two alternate strips. In order to minimize the contribution of c-rays, the experimental setup was modified as shown in Fig. 2b. The Cd double slit was removed and a second pyrolitic graphite crystal, C (0 0 2), was inserted at 300 mm distance from the exit of the Soller collimator. Two Cd slits (S1 and S2), 1.2 mm width and 20 mm length, at 350 mm reciprocal distance were inserted after the second graphite crystal. Finally, a third Cd slit (S3), 0.43 mm width and 20 mm length, was mounted on a translation stage, 100 lm minimum step, at 70 mm from the S2 slit and 50 mm from the detector that was maintained in a fixed position. The PHA spectra from a reference strip were collected at each translation position of the S3 slit. The translation step width was not uniform over the total 12 mm translation range and it was equal to 200 lm in the peak region. The spectra, integrated over the full energy range, are shown versus the translation position in Fig. 6. A comparison between the data in Figs. 4 and 6 points out that the main differences are observed on the tails of the intensity peaks. The extended intensity tails in Fig. 4a—c reveal the presence of energetic electrons that have longer ranges inside both Gd and Si. Electrons produced by c-ray absorption, with energies in excess of 250 keV can contribute to the signal from one strip, although having entered the Si sensor in points away from the reference strip. A second mechanism could be the direct detection of low-energy c-rays produced by collisions of the primary high-energy c-ray beam within the various materials. The hypothesis that the intensity tails mostly originate from absorption and collision mechanisms involving c-rays, is supported by the comparison of the experimental data collected in the two configurations. Indeed, by the double-crystal setup, a very clean neutron beam,
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Fig. 4. Energy-integrated PHA spectra versus translation position for the same strip as Fig. 3. (a) Data integrated from 10 to 70 keV. (b) Data integrated from 70 to 170 keV. (c) Data integrated from 170 to 1000 keV. The experimental data (dots) are compared with the results (full lines) of the Monte Carlo simulation of the detection process (see text).
Fig. 5. PHA spectra integrated from 10 to 70 keV versus translation position for two alternate strips of the 200 lm pitch portion of the sensor. The experimental data (dots) are compared with the results (full lines) of the Monte Carlo simulation of the detection process (see text).
depleted of the c-ray component in the direct beam, is produced on the detector. In both configurations, production of c-rays by neutron absorption in the Cd slits or in the aluminum detector box takes place, although there are differences in the incoming neutron flux. However, this effect seems to be of minor relevance, as the data of Fig. 6 show, although the role of the detector box and the electronics materials should be investigated in more detail. The observed tails and the broadening of the intensity curves of Fig. 4a—c with increasing the energy, substantially reflect the beam shape distribution. The experimental curves represent the convolution between the beam shape and the space resolution of the sensor at each translation position. The sensor resolution function, ideally defined as the
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Fig. 6. PHA spectra integrated over the full energy range versus translation position for a reference strip from the 400 lm pitch portion of the sensor. Experimental data (dots) collected in the configuration of Fig. 2b. The full line is the result of the Monte Carlo simulation of the detection process (see text). The curve resulting from the numerical convolution of the incoming beam profile with the sensor response function is represented by circles (see text).
detector response to a d-distribution beam profile, can be represented by the following approximate function: 1 1 R(x)" , (1) eV\ J #1 e\V> J#1 where 2* is the strip width and l is the average mean-free path of the electrons in silicon. The use of different optical elements in the two experimental configurations modifies the shape of the neutron beam. In the configuration of Fig. 2b, where the S3 Cd slit is translated and the sensor is fixed, the neutron beam profile at the sensor position was defined by the S1 and S3 Cd slits. The S2 Cd slit had substantially the effect of reducing the background. The resulting trapezoidal beam profile, 0.57 mm FWHM, was convoluted with the sensor response function using 2*"400 lm and assuming l "50 lm in Eq. (1). The convolution result is a curve with 0.62 mm FWHM which is shown in Fig. 6 in comparison with the experimental data. The quite good agreement between the calculated convolution and the experimental data is apparent and confirms the expected capabilities of the detector as to position resolution. In the configuration of Fig. 2a, where the detector is translated and the Cd double slit is fixed,
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neglecting the c-ray distribution, the neutron beam profile was defined by the Soller collimator coupled to the Cd double slit. Since the angular divergence of the Soller collimator was 0.5°, the neutron beam shape was dominated by the Soller collimation, resulting in a triangular distribution with &0.55 mm FWHM. However, in this case, being unknown the c-ray beam shape, no attempt was done to carry out the convolution between the beam shape and the sensor response function. The experimental data collected in this configuration were analyzed by means of a Monte Carlo simulation of the whole detection process.
4. Monte Carlo simulation An extended Monte Carlo analysis of the detection process was carried out in Ref. [7] pointing at the estimate of the neutron detection efficiency versus wavelength and converter thickness for various geometric configurations of the Si sensor and the Gd converter. To simulate the detector response in the present case, the strip configuration of the sensor is a necessary input data and allowance for the motion of the sensor relative to an incoming beam of variable size and shape must be considered. The Monte Carlo simulation would then provide the space resolution of the detector as a function of both the sensor geometry and the incoming beam characteristics. With reference to Fig. 7, the neutron flux inside the converter was modeled by the function U (x, y; j)"U (y) e\VIH, (2) G where k (j) is the linear attenuation coefficient of the converter. The incoming neutron flux was described by the function U (y) which accounts for the G (xz) section of the beam shape. In the linear strip detector, the relevant variation of the beam is only that along the y-direction perpendicular to the strip length. Several analytic options for U (y) were posG sible, namely, Gaussian, Lorentzian, triangular and trapezoidal functions. The tabulated values of the neutron cross sections in Gd from Ref. [13] were employed in the calculation. Only conversion electron production, upon neutron absorption in Gd, was considered as reaction channel, that is
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Fig. 7. Geometry of the Monte Carlo simulation: incoming beam along the x-direction, detector translation along the ydirection and strip length along the z-direction.
c-emission and production of secondary electrons in Gd were neglected. The quantum yield of the neutron/electron reaction was set to 0.8, according to Ref. [7]. The angular distribution of the electrons at the creation points was assumed to be isotropic. The electron creation points inside the Gd converter were randomly generated with a distribution proportional to U (y) along the y-direcG tion and an exponential decreasing distribution along the x-axis, corresponding to the neutron beam attenuation with depth according to Eq. (2). The conversion electron specific energy loss due to collision inside the Gd converter was calculated using the Bethe formula, adapted to the case of fast electrons [14]. The specific energy loss due to radiative processes, approximately given by ZE (MeV)/700 times the collisional loss, was neglected amounting to &2% of the collisional term for the most energetic electrons produced in Gd. The energy loss formula, applied to the conversion electron groups with initial kinetic energies centered at &70, &150 and &220 keV, was integrated to deduce the energy—distance relation. Being known, from the random sampling procedure, the coordinates of the creation points and the exit direction (escape angle), i.e. the path inside the converter, it was possible to deduce the energy of those electrons reaching the interface between Gd and Si and entering Si. From the residual electron energy, the total number of electron—hole pairs in Si can be deduced (&3.6 eV at RT are required to create an electron—hole pair). In order to deduce the space resolution of the sensor, it would be necessary
to follow the electron—hole pairs along their paths from the creation points to the closest electrode (strip). Therefore, the collisional energy loss formula was applied again to deduce, upon integration, the energy—distance relation inside Si for all the electrons escaped from Gd with a distributed initial kinetic energy. This amounts to know the number of the created electron—hole pairs as function of the distance. Being known the path direction and the total range of the electron inside Si, it was possible to deduce the space distribution of the released charge among different adjacent strips. The converter was assumed to be in contact with the sensor and the two configurations corresponding to the converter in transmission or in reflection were investigated. The Gd thickness was limited to 7 lm for transmission operation and the chosen value corresponds to the optimum thickness deduced from the Monte Carlo simulation of Ref. [7]. In reflection geometry, the Gd thickness can exceed this value and it was fixed to 250 lm, the actual value of the present neutron measurements. The sensor geometry was that of the test microstrip with 400 and 200 lm pitch regions, 300 lm depth and 20;20 mm area. Of course, the whole of these data, being an input set for the program, could be changed. Basically, two main calculations were carried out: the first was the simulation of the present measurements and the beam shape was consequently modeled; the second was the simulation of the space resolution obtained as the response of the sensor to a triangular incoming beam 10 lm wide. The experimental data collected in the configuration of Fig. 2b were simulated by using a trapezoidal distribution with 0.57 mm FWHM for the incoming beam. The calculated curve was normalized to the integral of the experimental data of Fig. 6 and it is also shown in Fig. 6. The excellent agreement with both the measured data and the convolution curve guarantees the reliability of the Monte Carlo simulation. The simulation of the data collected in the configuration of Fig. 2a was less direct, mainly because of the unknown c-contribution to the spectra. Although the overall shape of the experimental spectra could be reasonably reproduced assuming a 0.6 mm FWHM Lorentzian incoming beam, this result is physically doubtful. Indeed, the absence of
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intensity tails in the second set of data prevents from attributing this feature of the spectra to processes involving neutrons. Moreover, the neutron beam emerging from a Soller collimator, the effect of the fixed Cd double slit being negligible, would hardly be modeled by a Lorentzian curve. Under the assumption that the tails are produced by processes involving c-rays, the Monte Carlo program was used to separately simulate the response of the detector to both neutrons and c-rays. Triangular distributions were assumed for both neutron and c-ray beams and the energy of the primary c-rays was limited to 2 MeV. The calculated resolution curve, given by the weighted sum of the two simulation results, was fitted to the experimental data leaving the relative weigths as free parameters. The best fit to the measured data, tailored by the associate s, was obtained by modeling the incoming neutron beam by a triangular function with 0.55 mm FWHM and the c-ray beam (2 MeV initial energy) by a triangular function 1.5 mm FWHM. The c-ray beam was expected to have a broader profile, being less affected by the Soller collimation. The simulation results are shown in Fig. 4a—c in comparison with the experimental data over the three energy regions. In Fig. 8a and b the simulation curves, corresponding to the three energy regions, are shown versus position for both neutron and c-ray detection processes. Finally, making use of the so-optimized shape of the incoming beam, the response of the 200 lm pitch portion of the detector was simulated and the calculated curves are compared with the experimental data in Fig. 5. The overall good agreement between the data and the simulation results is an indication of the reliability of the program which was employed to simulate the response of a silicon sensor with variabile strip width to a narrow incoming neutron beam. The calculation was carried out at 1 As incoming neutron wavelength for a 10 lm FWHM triangular beam profile. The width of the sensor was ranging from 10 to 500 lm and the thickness of the backside natural Gd converter was 250 lm. The calculated curves obtained by integration of the energy-dependent spectra from 10 keV up to the maximum energy of secondary electrons are shown versus translation position and strip width in Fig. 9.
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Fig. 8. Monte Carlo results versus translation position simulating the response of the detector to neutrons (a) and c-rays (b). The calculated data, integrated from 10 to 70 keV (full lines), from 70 to 170 keV (short-dashed lines) and from 170 to 1000 keV (long-dashed lines) are shown in both cases. These simulation curves were employed to analyse the data collected in the configuration of Fig. 2a (see text).
The broadening introduced by the sensor response to the 10 lm wide incoming beam is quite small and of the order of 50 lm FWHM, independently of the width of the Si strip. The contribution to the counts collected on a reference strip due to charge released by electrons travelling far away the strip, which is accounted for by the Monte Carlo simulation, is low and does not produce a remarkable resolution degradation. The performances of the Si detector as to position resolution are quite satisfactory and could be exploited in those experimental cases where a fine resolution is required, like diffraction experiments with sample imaging as in in the case of residual stress measurements.
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Fig. 9. Monte Carlo simulation of the space resolution of the Si detector (see text).
References [1] ESS, in: J. Kiems, A.D. Taylor, J.L. Finney, H. Langeler, U. Steigenberger (Eds.), A Next Generation Neutron Source for Europe, vols. 1—3, 1997.
[2] R.K. Crawford, J. Neutron Res. 4 (1996) 97. [3] R.A. Boie, J. Fischer, Y. Inagaki, F.C. Merrit, H. Okuno, V. Radeka, Nucl. Instr. and Meth. 200 (1982) 533. [4] G. Cicognani, B. Guerard, A. Oed, Nucl. Instr. and Meth. A 392 (1997) 115. [5] N.J. Rhodes, M.W. Johnson, C.W.E. van Eijk, J. Neutron Res. 4 (1996) 129. [6] F. Cipriani, J.C. Castagna, C. Wilkinson, P. Oleinek, M.S. Lehmann, J. Neutron Res. 4 (1996) 79. [7] C. Petrillo, F. Sacchetti, O. Toker, N.J. Rhodes, Nucl. Instr. and Meth. A 378 (1996) 541. [8] C. Petrillo, F. Sacchetti, O. Toker, N.J. Rhodes, J. Neutron Res. 4 (1996) 65. [9] C. Petrillo, F. Sacchetti, N.J. Rhodes, M.W. Johnson, Technical Report RAL-TR-97-010, 1997. [10] E. Nygard, P. Aspell, P. Jarron, P. Weilhammer, K. Yoshioka, Nucl. Instr. and Meth. A 301 (1991) 506. [11] O. Toker, S. Masciocchi, E. Nygard, A. Rudge, P. Weilhammer, Nucl. Instr. and Meth. A 340 (1994) 572. [12] ISIS 97, The Rutherford Appleton Laboratory, ISIS Facility, Annual Report 1996-97, RAL-Tr-97-050, 1997. [13] C.D. Garber, R.R. Kinsey, Neutron Cross Sections, vol. II, 1976, BNL 325. [14] G.F. Knoll, Radiation Detection Measurements, Wiley, New York, 1989.
Space resolution of a Si/Gd microstrip as linear position-sensitive detector for thermal neutrons C. Petrillo *, F. Sacchetti , G. Maehlum , M. Mancinelli Istituto Nazionale per la Fisica della Materia, Unita& di Perugia, I-06123 Perugia, Italy Dipartimento di Fisica, Universita& di Perugia, Via A. Pascoli, I-06123 Perugia, Italy Received 10 August 1998; received in revised form 6 October 1998
Abstract A Si microstrip detector coupled to a Gd converter has been designed for operation as linear position-sensitive detector of thermal neutrons. Test measurements have been carried out on a first prototype. The measured space resolution is reported and discussed in comparison with the results of a Monte Carlo simulation. The space resolution of the Si microstrip detector, given by the response of the sensor to an ideal narrow incoming beam, has been also modeled by the Monte Carlo simulation program. 1999 Elsevier Science B.V. All rights reserved. PACS: 61.12.-q; 29.40.Wk; 29.40.Gx Keywords: Neutron scattering; Solid state detectors
1. Introduction The intensity gain expected from the high fluxes available at next generation neutron sources will result in higher data rates and will be exploited in higher-resolution experiments or smaller sample investigations [1]. The time structure of the neutron beam in a pulsed source, together with the effective intensity gain, will demand an improvement in detector capabilities like the maximum instantaneous rate of data acquisition and/or the space resolution in position sensitive devices.
* Correspondence address: Dipartimento di Fisica, Universita’ di Perugia, Via A. Pascoli, 06123 Perugia, Italy. Tel.: (39) 75 585 3021; e-mail: [email protected].
Moreover, improved performances in both shape flexibility and total coverage surface will be relevant for fully exploiting the neutron source possibilities by new spectrometer designs. Current capabilities and possible improvements of traditional gas detectors for thermal neutrons have been reviewed in Ref. [2] where the limitations on position resolution and maximum data rate achievable in gas detectors are discussed. The best position resolution values reported for the Multi-Wire-Proportional-Chamber (MWPC) detector range from 1.3 to 2 mm Full-Width at HalfMaximum (FWHM) in both dimensions on a 500;500 mm detector [2,3], whereas data rates exceeding 10 s\ mm\ are expected by use of Micro Strip Gas Chambers (MSGC) [4]. In current operation, proportional gas counters yield typical
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values of a few millimeters in position resolution at usable data rates ranging from 10 to 10 s\ over the total area. Approximately the same performances are offered by scintillator detectors. The characteristics of ZnS(Ag)/Li-based detectors, routinely operated on many spectrometers at ISIS, are reviewed in Ref. [5]: a position resolution as good as 3;3 mm at usable data rates of &4;10 over the pixel area is reported for the 4096-pixel SXD detector. Higher resolutions are obtained by image plates [6] or neutron-sensitive Charge Coupled Device (CCD), typically 100 lm. These detectors, however, cannot be employed as realtime counters and their use is confined to specific applications at steady sources and it is ruled out at pulsed sources when the time-of-flight techniques are exploited. Silicon strip detectors are known for their unrivalled space resolution at high counting rates and they are widely employed as particle trackers in high-energy physics. The performances achievable by Si devices in thermal neutron detection have been recently investigated [7—9] with focus on neutron detection efficiency, effective dead time, c-ray rejection and stability. Measurements were carried out on simple detector prototypes whose design was optimized to fit with the parameters to be tested. Neutron data rates up to 5;10 s\ mm\ were found to be accessible [7] with the prototype detectors coupled to VLSI (Very Large Scale Integration) chips [10,11] as multi-channel charge-sensitive preamplifiers, while only preliminary space resolution measurements were carried out [7]. In the present paper, the space resolution of a microstrip detector for thermal neutrons is investigated. The prototype was designed as a small basic unit of a modular expandable linear Position-Sensitive Detector (PSD) for coverage of larger areas. For the Si detector to be a really useful neutron PSD, relatively large areas need to be covered and this can be achieved by the optimized layout of the maximum size available sensor unit. Moreover, even for the single unit, the parallel readout of the rather large number of output channels requires a proper fanout of the signal from the VLSI frontend electronics to the conventional electronics. The space resolution measurements on the
prototype detector are reported and compared with the results of a Monte Carlo simulation.
2. Detector prototype The silicon sensor was a double-sided microstrip with 50 lm pitch and 20;20 mm size operated as a single-sided microstrip for being a linear position sensitive detector. Signals from the front side (p-side) strips were collected while the backplane (n-side) strips were electrically connected together to give a common signal through a resistive load. Considering that not extremely high resolutions are required for normal operation as neutron detector, groups of strips were connected together via ultrasonics microbonding. Each group was coupled to a channel of the readout chip by integrated capacitors having 16.7;6.6 mm size and 100 lm pitch. In Fig. 1a the pattern of the connections is schematically shown. On each side of the sensor, alternate strips are connected to the capacitors thus matching the 100 lm pitch of the capacitor itself. Grouping of four and two alternate strips is then obtained through a common bonding on the same bonding pad. The signal from the group is addressed to one channel of the readout chip. By this configuration, an effective 400 lm pitch sensor, with the central portion at 200 lm pitch, was obtained. The possibility of varying the pitch between 200 and 400 lm on the same sensor was preferred in this prototype version to test the performances obtainable with different configurations in terms of efficiency and noise. Of course, grouping of strips increases the sensor capacitance and, as a consequence, the effective input noise. Nonetheless, the very low-noise preamplifiers employed in the present prototype allowed for safe operation of the sensor with strips connected in groups of four elements. The readout chip, 1;3 mm size, was the 8-channel low-noise charge-sensitive preamplifier and shaper VA (Ide As, Norway). For the detector to be space expandable, the VA chips were mounted on both sides of the sensor. With the strips running vertically, several of these detector units, or similarly designed systems, could be put close together minimising the hindrance of the frontend
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Fig. 1. Schematic of the microstrip detector: (a) strip and connection pattern; (b) detector board layout.
electronics. The first prototype was assembled using 8 VA chips (64 output channels) in order to independently read out the 200 lm pitch central portion of the sensor on both sides. The resulting layout is schematically shown in Fig. 1b. Specifically, the upper (16) and bottom (16) groups of strips (400 lm pitch) were connected to the VAs on the opposite sides of the sensor with the central portion of the sensor (200 lm pitch) independently read out on each side (32 groups, 16 groups per side). The final detector will have a similar design but with constant pitch and one output channel per strip group. The neutron converter was the already tested natural gadolinium which was mounted in contact with the backplane of the sensor, ensuring at the same time the electrical connection of all the strips on the ohmic n-side. Use of the detector in reflection geometry with respect to the neutron beam does not limit the maximum thickness of the Gd converter. A commercial Gd plate, 250 lm thick-
ness, 25;25 mm size was used (Goodfellow, UK). The less energetic line of the conversion electrons following the neutron capture in Gd is observed at &70 keV giving rise to the production of &20000e\, i.e. &3.2 fC released charge inside the 300 lm thick Si sensor. The VA chip has a gain of &28 mV/fC and was set for a peaking time of 1 ls. This chip can only drive a high impedance load, therefore, to increase the gain and to buffer the chip signal, a further amplification stage based on conventional operational amplifiers (quad-amplifiers OP467) was assembled. The 64 output signals from the VA chips were connected through a short flat cable to the amplification board. Both the board allocating sensor and frontend electronics and the auxiliary amplification board were contained inside the same aluminum box. Bias voltage to the sensor and power to the chips were supplied by an external power supply. The 64 amplified signals could be sent to a 64-channels parallel data acquisition
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electronics (like the standard DAE at ISIS [12]). However, for the present test measurements, the 64 signals were separately sent to a PC equipped with a Scope card (ADC, 100 MHz, 8-Bit; Matec Instruments, USA) and a PHA (Pulse Height Analyzer) card (ADC, 8000 channels; Silena, Italy) for collection of single pulses or the full PHA spectrum from the strips. The operation of the detector was tested by using both a b-emitter source (Sr) and monochromatic X-rays from Ag K . a 3. Neutron measurements The prototype detector was tested on the thermal neutron beam of the TRIGA reactor (ENEA-CRE Casaccia, Rome, Italy). Two different experimental setups, schematically shown in Fig. 2a and b, were employed, the main difference being the amount of
c-rays in the incoming beam. No neutron or c-ray shielding was employed in all the experimental tests. In the experimental configuration of Fig. 2a, the white moderated neutron beam emerging from the reactor was monochromatized through the (0 0 2) Bragg reflection in pyrolitic graphite, resulting in the incoming wavelength j"1.42 As . A vertical Soller collimator, 350 mm length and 3 mm slit separation, giving an angular divergence of 0.5°, was inserted after the monochromator. The size of the exit hole was defined by an adjustable cadmium double-slit arranged to 0.31 mm width and 15 mm length. The detector was mounted on a translation stage, 50 lm minimum step, at 28 mm distance from the exit hole with the strips running parallel to the slit length. In such a configuration the incoming beam was rather intense but strongly contaminated by medium energy (1—2 MeV) c-rays, those present in the neutron beam before the
Fig. 2. Schematic of the experimental setup. (a) Detector on the direct beam and mounted on a translation stage. (b) Detector on the double-crystal beam and maintained in a fixed position. S1, S2: Cd slits, 1.2 mm width, 20 mm length. S3: Cd slit, 0.43 mm width, 20 mm length. The slit S3 was mounted on a translation stage.
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monochromator as well as those produced by neutron absorption in the Cd slit. The c-ray beam had therefore a structure broader than that of the neutron beam. The presence of this remarkable c-ray field was exploited to investigate the effect produced on the performance of the detector. The neutron measurements were carried out by translating the detector in steps of 0.1 mm in front of the incoming beam and collecting the PHA spectra at each translation position. The procedure was repeated for various strips. The reason for collecting the full PHA spectra is that the energy dependence of the space resolution can be checked together with the contribution brought about by c-rays having an energy in excess of &250 keV, that is above the maximum energy of conversion electrons. An overall view of the space resolution dependences is presented in Fig. 3 where the PHA spectra collected for one strip are shown as function of both the energy and the translation position. The data in Fig. 3 were normalized to the same value of the intensity integral over the translation position at each energy channel. The normalization was applied in order to enhance the change of the peak width. From Fig. 3, the broadening of the curves versus the position, with increasing the electron energy, is apparent. Therefore, the PHA spectra, measured at each translation position, were integrated over three different energy regions, namely from 10 to 70 keV, from 70 to 170 keV and from 170 to 1000 keV. In Fig. 4a—c the data obtained after
Fig. 3. PHA spectra versus energy and translation position for a reference strip from the 400 lm pitch portion of the sensor. Data collected in the experimental configuration of Fig. 2a.
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integration over these three regions are shown as a function of the position for the same reference strip. The increase in width with increasing the energy is apparent from these figures. Finally, in Fig. 5 the spectra integrated from 10 to 70 keV of two alternate strips belonging to the 200 lm pitch portion of the sensor are shown versus the translation position. As expected, the distance between the peak positions is 400 lm and this figure is representative of the effect expected from a simultaneous readout of two alternate strips. In order to minimize the contribution of c-rays, the experimental setup was modified as shown in Fig. 2b. The Cd double slit was removed and a second pyrolitic graphite crystal, C (0 0 2), was inserted at 300 mm distance from the exit of the Soller collimator. Two Cd slits (S1 and S2), 1.2 mm width and 20 mm length, at 350 mm reciprocal distance were inserted after the second graphite crystal. Finally, a third Cd slit (S3), 0.43 mm width and 20 mm length, was mounted on a translation stage, 100 lm minimum step, at 70 mm from the S2 slit and 50 mm from the detector that was maintained in a fixed position. The PHA spectra from a reference strip were collected at each translation position of the S3 slit. The translation step width was not uniform over the total 12 mm translation range and it was equal to 200 lm in the peak region. The spectra, integrated over the full energy range, are shown versus the translation position in Fig. 6. A comparison between the data in Figs. 4 and 6 points out that the main differences are observed on the tails of the intensity peaks. The extended intensity tails in Fig. 4a—c reveal the presence of energetic electrons that have longer ranges inside both Gd and Si. Electrons produced by c-ray absorption, with energies in excess of 250 keV can contribute to the signal from one strip, although having entered the Si sensor in points away from the reference strip. A second mechanism could be the direct detection of low-energy c-rays produced by collisions of the primary high-energy c-ray beam within the various materials. The hypothesis that the intensity tails mostly originate from absorption and collision mechanisms involving c-rays, is supported by the comparison of the experimental data collected in the two configurations. Indeed, by the double-crystal setup, a very clean neutron beam,
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Fig. 4. Energy-integrated PHA spectra versus translation position for the same strip as Fig. 3. (a) Data integrated from 10 to 70 keV. (b) Data integrated from 70 to 170 keV. (c) Data integrated from 170 to 1000 keV. The experimental data (dots) are compared with the results (full lines) of the Monte Carlo simulation of the detection process (see text).
Fig. 5. PHA spectra integrated from 10 to 70 keV versus translation position for two alternate strips of the 200 lm pitch portion of the sensor. The experimental data (dots) are compared with the results (full lines) of the Monte Carlo simulation of the detection process (see text).
depleted of the c-ray component in the direct beam, is produced on the detector. In both configurations, production of c-rays by neutron absorption in the Cd slits or in the aluminum detector box takes place, although there are differences in the incoming neutron flux. However, this effect seems to be of minor relevance, as the data of Fig. 6 show, although the role of the detector box and the electronics materials should be investigated in more detail. The observed tails and the broadening of the intensity curves of Fig. 4a—c with increasing the energy, substantially reflect the beam shape distribution. The experimental curves represent the convolution between the beam shape and the space resolution of the sensor at each translation position. The sensor resolution function, ideally defined as the
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Fig. 6. PHA spectra integrated over the full energy range versus translation position for a reference strip from the 400 lm pitch portion of the sensor. Experimental data (dots) collected in the configuration of Fig. 2b. The full line is the result of the Monte Carlo simulation of the detection process (see text). The curve resulting from the numerical convolution of the incoming beam profile with the sensor response function is represented by circles (see text).
detector response to a d-distribution beam profile, can be represented by the following approximate function: 1 1 R(x)" , (1) eV\ J #1 e\V> J#1 where 2* is the strip width and l is the average mean-free path of the electrons in silicon. The use of different optical elements in the two experimental configurations modifies the shape of the neutron beam. In the configuration of Fig. 2b, where the S3 Cd slit is translated and the sensor is fixed, the neutron beam profile at the sensor position was defined by the S1 and S3 Cd slits. The S2 Cd slit had substantially the effect of reducing the background. The resulting trapezoidal beam profile, 0.57 mm FWHM, was convoluted with the sensor response function using 2*"400 lm and assuming l "50 lm in Eq. (1). The convolution result is a curve with 0.62 mm FWHM which is shown in Fig. 6 in comparison with the experimental data. The quite good agreement between the calculated convolution and the experimental data is apparent and confirms the expected capabilities of the detector as to position resolution. In the configuration of Fig. 2a, where the detector is translated and the Cd double slit is fixed,
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neglecting the c-ray distribution, the neutron beam profile was defined by the Soller collimator coupled to the Cd double slit. Since the angular divergence of the Soller collimator was 0.5°, the neutron beam shape was dominated by the Soller collimation, resulting in a triangular distribution with &0.55 mm FWHM. However, in this case, being unknown the c-ray beam shape, no attempt was done to carry out the convolution between the beam shape and the sensor response function. The experimental data collected in this configuration were analyzed by means of a Monte Carlo simulation of the whole detection process.
4. Monte Carlo simulation An extended Monte Carlo analysis of the detection process was carried out in Ref. [7] pointing at the estimate of the neutron detection efficiency versus wavelength and converter thickness for various geometric configurations of the Si sensor and the Gd converter. To simulate the detector response in the present case, the strip configuration of the sensor is a necessary input data and allowance for the motion of the sensor relative to an incoming beam of variable size and shape must be considered. The Monte Carlo simulation would then provide the space resolution of the detector as a function of both the sensor geometry and the incoming beam characteristics. With reference to Fig. 7, the neutron flux inside the converter was modeled by the function U (x, y; j)"U (y) e\VIH, (2) G where k (j) is the linear attenuation coefficient of the converter. The incoming neutron flux was described by the function U (y) which accounts for the G (xz) section of the beam shape. In the linear strip detector, the relevant variation of the beam is only that along the y-direction perpendicular to the strip length. Several analytic options for U (y) were posG sible, namely, Gaussian, Lorentzian, triangular and trapezoidal functions. The tabulated values of the neutron cross sections in Gd from Ref. [13] were employed in the calculation. Only conversion electron production, upon neutron absorption in Gd, was considered as reaction channel, that is
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Fig. 7. Geometry of the Monte Carlo simulation: incoming beam along the x-direction, detector translation along the ydirection and strip length along the z-direction.
c-emission and production of secondary electrons in Gd were neglected. The quantum yield of the neutron/electron reaction was set to 0.8, according to Ref. [7]. The angular distribution of the electrons at the creation points was assumed to be isotropic. The electron creation points inside the Gd converter were randomly generated with a distribution proportional to U (y) along the y-direcG tion and an exponential decreasing distribution along the x-axis, corresponding to the neutron beam attenuation with depth according to Eq. (2). The conversion electron specific energy loss due to collision inside the Gd converter was calculated using the Bethe formula, adapted to the case of fast electrons [14]. The specific energy loss due to radiative processes, approximately given by ZE (MeV)/700 times the collisional loss, was neglected amounting to &2% of the collisional term for the most energetic electrons produced in Gd. The energy loss formula, applied to the conversion electron groups with initial kinetic energies centered at &70, &150 and &220 keV, was integrated to deduce the energy—distance relation. Being known, from the random sampling procedure, the coordinates of the creation points and the exit direction (escape angle), i.e. the path inside the converter, it was possible to deduce the energy of those electrons reaching the interface between Gd and Si and entering Si. From the residual electron energy, the total number of electron—hole pairs in Si can be deduced (&3.6 eV at RT are required to create an electron—hole pair). In order to deduce the space resolution of the sensor, it would be necessary
to follow the electron—hole pairs along their paths from the creation points to the closest electrode (strip). Therefore, the collisional energy loss formula was applied again to deduce, upon integration, the energy—distance relation inside Si for all the electrons escaped from Gd with a distributed initial kinetic energy. This amounts to know the number of the created electron—hole pairs as function of the distance. Being known the path direction and the total range of the electron inside Si, it was possible to deduce the space distribution of the released charge among different adjacent strips. The converter was assumed to be in contact with the sensor and the two configurations corresponding to the converter in transmission or in reflection were investigated. The Gd thickness was limited to 7 lm for transmission operation and the chosen value corresponds to the optimum thickness deduced from the Monte Carlo simulation of Ref. [7]. In reflection geometry, the Gd thickness can exceed this value and it was fixed to 250 lm, the actual value of the present neutron measurements. The sensor geometry was that of the test microstrip with 400 and 200 lm pitch regions, 300 lm depth and 20;20 mm area. Of course, the whole of these data, being an input set for the program, could be changed. Basically, two main calculations were carried out: the first was the simulation of the present measurements and the beam shape was consequently modeled; the second was the simulation of the space resolution obtained as the response of the sensor to a triangular incoming beam 10 lm wide. The experimental data collected in the configuration of Fig. 2b were simulated by using a trapezoidal distribution with 0.57 mm FWHM for the incoming beam. The calculated curve was normalized to the integral of the experimental data of Fig. 6 and it is also shown in Fig. 6. The excellent agreement with both the measured data and the convolution curve guarantees the reliability of the Monte Carlo simulation. The simulation of the data collected in the configuration of Fig. 2a was less direct, mainly because of the unknown c-contribution to the spectra. Although the overall shape of the experimental spectra could be reasonably reproduced assuming a 0.6 mm FWHM Lorentzian incoming beam, this result is physically doubtful. Indeed, the absence of
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intensity tails in the second set of data prevents from attributing this feature of the spectra to processes involving neutrons. Moreover, the neutron beam emerging from a Soller collimator, the effect of the fixed Cd double slit being negligible, would hardly be modeled by a Lorentzian curve. Under the assumption that the tails are produced by processes involving c-rays, the Monte Carlo program was used to separately simulate the response of the detector to both neutrons and c-rays. Triangular distributions were assumed for both neutron and c-ray beams and the energy of the primary c-rays was limited to 2 MeV. The calculated resolution curve, given by the weighted sum of the two simulation results, was fitted to the experimental data leaving the relative weigths as free parameters. The best fit to the measured data, tailored by the associate s, was obtained by modeling the incoming neutron beam by a triangular function with 0.55 mm FWHM and the c-ray beam (2 MeV initial energy) by a triangular function 1.5 mm FWHM. The c-ray beam was expected to have a broader profile, being less affected by the Soller collimation. The simulation results are shown in Fig. 4a—c in comparison with the experimental data over the three energy regions. In Fig. 8a and b the simulation curves, corresponding to the three energy regions, are shown versus position for both neutron and c-ray detection processes. Finally, making use of the so-optimized shape of the incoming beam, the response of the 200 lm pitch portion of the detector was simulated and the calculated curves are compared with the experimental data in Fig. 5. The overall good agreement between the data and the simulation results is an indication of the reliability of the program which was employed to simulate the response of a silicon sensor with variabile strip width to a narrow incoming neutron beam. The calculation was carried out at 1 As incoming neutron wavelength for a 10 lm FWHM triangular beam profile. The width of the sensor was ranging from 10 to 500 lm and the thickness of the backside natural Gd converter was 250 lm. The calculated curves obtained by integration of the energy-dependent spectra from 10 keV up to the maximum energy of secondary electrons are shown versus translation position and strip width in Fig. 9.
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Fig. 8. Monte Carlo results versus translation position simulating the response of the detector to neutrons (a) and c-rays (b). The calculated data, integrated from 10 to 70 keV (full lines), from 70 to 170 keV (short-dashed lines) and from 170 to 1000 keV (long-dashed lines) are shown in both cases. These simulation curves were employed to analyse the data collected in the configuration of Fig. 2a (see text).
The broadening introduced by the sensor response to the 10 lm wide incoming beam is quite small and of the order of 50 lm FWHM, independently of the width of the Si strip. The contribution to the counts collected on a reference strip due to charge released by electrons travelling far away the strip, which is accounted for by the Monte Carlo simulation, is low and does not produce a remarkable resolution degradation. The performances of the Si detector as to position resolution are quite satisfactory and could be exploited in those experimental cases where a fine resolution is required, like diffraction experiments with sample imaging as in in the case of residual stress measurements.
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Fig. 9. Monte Carlo simulation of the space resolution of the Si detector (see text).
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