- Email: [email protected]
A CdTe position sensitive spectrometer for hard X- and soft γ-ray polarimetry
Nuclear Instruments and Methods in Physics Research A 477 (2002) 567–573
A CdTe position sensitive spectrometer for hard X- and soft g-ray polarimetry E. Carolia,*, J.B. Stephena, W. Dusia, G. Bertucciob, M. Sampietrob, A.J. Birdc, A.J. Deanc, R.M. Curado da Silvad, P. Siffertd, V. Regleroe, W. Yuf, C. Zhangf a Istituto TESRE/CNR,Via Gobetti 101, 40129 Bologna, Italy Dip. di Elettronica ed Informazione, Politecnico, Piazza L. Da Vinci 32, 20133 Milano, Italy c Physics and Astronomy Department, University of Southampton, SO17 1BJ Southampton, UK d Laboratoire PHASE/CNRS, 67037 Strasbourg, France e Dpto. de Astronomia y Astrofisica, University of Valencia, 46100 Burjassot, Spain f Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, China b
Abstract Coded Imager and Polarimeter for High Energy Radiation (CIPHER) is a hard X- and soft g- ray spectroscopic and polarimetric coded mask telescope based on an array of Cadmium telluride microspectrometers. The position sensitive detector (PSD) will be arranged in 4 modules of 32 32 crystals, each of 2 2 mm2 cross-section and 10 mm thickness giving a total active area of about 160 cm2, operating over a wide energy range (B10 keV to 1 MeV). Each PSD module is obtained by aligning 32 linear arrays of micro-detectors each also containing the integrated analog front end electronics on a thin ceramic layer. The CIPHER instrument will be proposed for a balloon experiment, both in order to assess the performance of such an instrumental concept for a small/medium size satellite (or an external ISS-alpha payload) survey mission and to perform an innovative high energy polarisation measurement. Herein we describe the CIPHER position sensitive spectrometer design, together with current development on CdTe detectors array and on a low noise and low power consumption ASIC for the analog front end electronics. Furthermore we present the expected performance in terms of image and spectral quality (spatial and energy resolution) and polarimetric capabilities for such a detector. r 2002 Elsevier Science B.V. All rights reserved. PACS: 95.55.Ka; 95.85.Pw; 95.75.Hi Keywords: CdTe spectrometers; Hard X- and soft g-rays; Polarimetry
1. Introduction Polarisation studies could offer astrophysicists very important information about the emission mechanisms and the physical conditions of cosmic *Corresponding author. Tel.: +39-51-6398-678; fax: +3951-6398-723. E-mail address: [email protected] (E. Caroli).
sources because almost all mechanisms that generate high-energy emission involve strong magnetic fields and lead to the production of polarised photons. Even if polarisation measurements in lower energy bands have been extremely useful, and although telescopes such as the COMPTEL instrument on the Compton Gamma Ray Observatory are theoretically able to perform polarisation measurements the sensitivity is such that no
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 9 2 4 - 6
568
E. Caroli et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 567–573 Table 1 Then main characteristics of CIPHER PSD
(a)
(b) Fig. 1. (a) Shows one of the four CIPHER PSD basic modules, (b) the drawing represents a possible mechanical structure for the four module PSD. The hatched box shows the position of the digital front end electronics.
actual measurements have been performed at energies greater than about 10 keV [1]. In order to fulfil this scientific objective, we briefly describe the design and the current development of a modular position sensitive detector based on CdTe arrays suitable for polarisation measurements on a balloon borne coded mask telescope operative in the range between B10 keV and 1 MeV [2].
2. The position sensitive detector The position sensitive detector (PSD) of CIPHER is made of a matrix of Cadmium Telluride micro-spectrometers. Taking advantage of current CdTe technology, the proposed detector is highly modular. The dimensions of each CdTe crystal are 2 2 10 mm3. These units are used in the
Active pixel size Geometrical pixel size Linear module Matrix module Matrix module number PSD active area PSD geometrical area Detector thickness CdTe mass Linear module support Matrix module container Detector frame PSD overall height
2 2 mm2 2.6 2.1 mm 32 CdTe micro-crystals 32 32 pixels 22 164 cm2 18 15 cm2=270 cm2 10 mm 1 kg Al2O3 (0.3 mm thick) Aluminium Aluminium or carbon fiber 5.5 cm
configuration in which the optical axis is orthogonal to the charge collecting. Using this configuration it is possible to increase the photon detection efficiency at high energies while limiting the spectroscopic performance loss due to charge trapping in the material. These micro-spectrometers are assembled on thin (300 mm) ceramic (Al2O3) plates in linear modules that contain 32 units (with a pitch of 2.1 mm) together with integrated low noise and low power consumption analogue readout electronics as well as the bias circuits (Fig. 1a) The linear modules are packed together at 2.6 mm spacing in a matrix module containing 32 32 CdTe units inside an aluminium case. Below the matrix module, two layers are foreseen containing the hybrid front-end electronics (FEE) with multiplexer and ADCs for the 1024 channels. The matrix module is therefore a complete and independent detector that can be tested and calibrated separately. Finally the CIPHER PSD is formed of 4 matrix modules for a total sensitive area of 164 cm2 (Fig. 1b). The matrix modules are supported by a metallic (Al) or carbon fibre grid. The grid sides provide the mechanical interface for the active veto shield. In Table 1 the main characteristics of the PSD are reported.
3. The detector performance Cadmium telluride is a room temperature semiconductor compound that has been studied
569
E. Caroli et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 567–573 57
4000
Co Collimated Source
3500
Counts
3000 2500 2000 1500 1000 500 0 0
50
100
150
200
250
300
350
Channel 22
Na uncollimated source
10000
Counts
for 20 years as a radiation detector and in the last decade has started to be proposed for space applications in X- and g-ray astronomy [3–6]. CdTe crystals are well suited to the construction of fine spatial resolution position sensitive detectors both using micro-strip (or pixellation) techniques and micro-spectrometer arrays [7]. Even though crystal growing technology has made large advances and has allowed the introduction of a new cadmium telluride compound (CdZnTe), the spectroscopic capabilities of this material is still limited by charge trapping effects. These effects do not allow an increase of distance between the charge collecting electrodes above a few mm, even if, currently, there are promising results in single charge carrier thick CdZnTe detector development [8]. In the traditional use of CdTe detectors the photons are incident on one of the electrodes, thus limiting the absorption thickness and the operative range up to 200–300 keV. Several years ago, the TeSRE group proposed a geometry in which the charge collection field is orthogonal to the optical axis of the detector (PTF: planar transverse field) [9]. In this configuration the charge collection distance is independent from the photon absorption thickness, making this kind of detector suitable for higher energy applications (up to the MeV band). The CdTe micro-crystal configuration proposed for the realisation of the CIPHER PSD has been tested for a long time in the laboratory; a pair of spectra obtained with 2 2 10 mm3 PTF micro-spectrometers are shown in Fig. 2 [10]. These measures has been obtained by selecting events with short rise times: either by collimating a source over a B0.5 mm wide region close to the cathode or by using rise time selection electronics [11]. Using these techniques a typical energy resolution of B5% at 122 keV and 2% at 511 keV are achievable. We are confident that similar results will be obtained implementing an energy compensation based on double shaping technique without significant efficiency loss [12]. In order to evaluate the imaging performance of the CIPHER telescope the knowledge of the point spread function of the PSD is required. The geometrical size of the pixel is 2.1 2.6 mm2, while the PSF is dependent on photon energy and
1000
100 0
100
200
300
400
500
600
700
Channel
Fig. 2. Radiation source spectra obtained in laboratory tests using 2 2 10 mm PTF CdTe spectrometers. These results was obtained with a selection of short rise time signals (we expect similar results by compensating signals with double shaping technique, see Section 5).
incidence angle. The results that we have obtained with numerical simulations indicate that the PSF of the CIPHER PSD does not have a strong dependence on photon energy: the PSF is in general sharper than a Gaussian and is almost independent of energy up to 2 MeV for on-axis sources with a ratio between PSF peak and tails ranging from 120 to 30 (from 0.06 to 1.8 MeV). This energy independence is mainly due to the limited thickness of the PSD, which means that scattered events constitute only a small fraction of the total recorded events (Table 2) [13] and to the high degree of pixellisation that allows an efficient reconstruction of the incidence pixel. The shape of
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E. Caroli et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 567–573
Table 2 CIPHER total and double event efficiencya E (keV)
Total efficiency
Double events efficiency
20 50 100 200 500 1000
1 1 0.99 0.83 0.41 0.29
0 0.036 0.064 0.170 0.118 0.074
results of numerical simulations indicates that this effect becomes of some importance only for angles greater than 201 and for energies above 500 keV. Therefore we can conclude that the PSF is equal to the pixel geometrical size inside the foreseen CIPHER field of view (B101).
4. Polarimetric capabilities
a
The errors on values are less 2%. (This results have been obtained with Monte Carlo simulation. The double events (events that hits two CdTe pixels) efficiency is directly related to the detector polarimetric capabilities.)
(a)
(c)
(b)
(d)
Fig. 3. The CIPHER PSF at 500 keV: Counts map (A) and reconstructed pixel map (B) for a source on axis; (C) and (D) same maps for an incidence angle of 351. The reconstruction of the incidence pixel in multiple events is obtained with an energy weighted mean.
the PSF is also almost independent (within 10%) from the criterion adopted for the incidence position reconstruction of multiple events (events that hit more than one CdTe element): e.g. mean weighted by energy deposits, or by selection of the pixel with the minimum or maximum energy deposit. In any case, the tails extend at maximum over 4 pixels. Of course the broadening of the spatial response becomes more important as the photon incidence angle increases (Fig. 3). The
The use of a highly pixellated detector is ideal for use as a polarimeter and the CIPHER PSD design, with its high sensitivity, is particularly suited for this purpose. In order to investigate the efficiency of this detector design for polarimetric measurements we have calculated the response of the instrument to a 100% polarized beam of photons in order to obtain the Q polarimetric modulation factor [14]. This is obtained by integrating the formula for the Compton polarimetric differential cross-section: Z ds S ¼ dO dO Z 2 2 r0 e 1 þ e 2 sin2 y cos2 j dO ¼ 2 e where e is the ratio of the Compton scattered energy to that of the incoming photon, r0 is the classical electron radius, y is the angle between the incident and scattered photon direction and j is the azimuthal angle between the scattered photon and the electric vector of the incident photon. The integration takes place over the solid angles defined by the geometry of the detection plane in such a way as to be able to determine the Q-factor defined as N> N8 Q¼ N> þ N8 where N> and N: are the count rates in orthogonal detectors in the X/Y plane. In Fig. 4 we show the Q-factor as a function of photon energy from 100 keV to 1 MeV, showing that this design is very competitive with other proposed instrumentation [1]. In particular this number is related to double-hit event efficiency that for the CIPHER typically ranges from B8% to 16% in the 0.2–1. MeV energy band (Table 2).
E. Caroli et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 567–573
571
5. Current development
Q Factor
0.5
0.4
0.3
100
1000
Energy (keV)
Fig. 4. The Q-factor evaluated for the CIPHER position sensitive detector.
In this section we briefly describe some activities and projects that in particular the TESRE and the Politecnico of Milan groups are performing on the development of this PSD detector in the framework of the CIPHER telescope collaboration. In particular, we have built some linear array prototypes (Fig. 5). These prototype are reduced versions of that proposed for the CIPHER PSD containing only 8 CdTe micro-spectrometers. These prototypes are realised with hybrid techniques, in which only the charge sensitive preamplifier (CSP) is monolithic, while the bias, filter and shaper are made with SMD techniques. The CSP itself is a prototype designed for silicon high
Fig. 5. The pictures show a linear array of 8 CdTe of 2 2 10 mm3 on a 0.5 mm thick ceramic layer. In this prototype each CdTe spectrometer (C) is read out by an integrated CSP (under black coating, A). On the opposite side (B) the SMD filters, bias and shaping circuits for each channel are visible.
Fig. 6. A small matrix of 5 5 CdTe micro-detector that can be used to test the polarimetric capabilities of the CIPHER PSD concept. Each 2 2 10 mm3 crystal is used in PTF configuration and operated at bias up to 150. The CdTe array is mounted on a ceramic disk and is surrounded by the 25 charge preamplifier (Katsura CS507). The aluminium box contains the bias (high and low) distribution board. On the box lateral surfaces are positioned the signal and power connectors as well as the ventilation hole. The energy threshold is around 25 keV.
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E. Caroli et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 567–573
energy physics application at the University of Pavia. This linear module will be used in the near future both to asses the real performance of such a position sensitive spectrometer under laboratory conditions and to tune some of the parameters of the low noise and power consumption ASIC that is under development at the Politecnico of Milano. At the same time we are setting up a multichannel test equipment to perform tests on a small CdTe matrix (Fig. 6) to verify the achievable spatial resolution of the detector as function of energy and incidence angle and to assess the polarimetric capability of this kind of position sensitive detector. The requirement for the CIPHER detector to be sensitive down to B10 keV implies the use of a low noise front-end amplifier, at or below 500 electrons r.m.s. for a detector capacitance of about 1 pF. This performance should be reached with minimum power dissipation (o 1 mW/channel) due to the high pixellisation of the detector and to power resource limitations as well as thermal balance problems in space instrumentation. An answer to these requirements will be given by the ELBA (‘‘ELettronica integrata a BAssa potenza’’: low power integrated electronics) project. This low power and low noise characteristic imposed on the front-end electronics has led to an innovative scheme for the preamplifier implemented using BiCMOS technology with the introduction of an active current conveyor stage with a small resistor that acts as a high value feedback resistance [15,16]. The adopted circuit will manage signal over two orders of magnitude amplitude dynamic range with a linearity, inferred from first test measurements, below 1%. The whole amplifier is powered with a single supply voltage of 4 V and dissipates about 550 mW. Extensive experimental tests of the preamplifier and the shaper are presently underway. In Table 3 are shown the main performance and characteristics of the ELBA multichannel chip. Futhermore, the ELBA ASIC design will allow a signal processing suitable for compensating for trapping effects by a double filtering stage for each channel. One filter will operate at a relatively short signal shaping time (from 100 to 250 ns) while the other filter, working in parallel on the same channel, will have a longer
Table 3 ELBA ASICS main characteristics Channel per ASIC Dynamic range Linearity Shaping type Shaping times per channel Peaking time Single power supply Power dissipation (PA+Shaper) Equivalent noise charge Equivalent energy resolution Energy threshold on CdTe
8–16 10–1000 keV o0.3% (Simulation) Unipolar semi-Gaussian 2 (200 ns and 2 ms) 1.5 ms +4 V 590 mW per channel 455e (r.m.s.) 4.5 keV (FWHM) B10 keV (5s)
shaping time (from 1 to 3 ms). The knowledge of the ratio between the amplitudes of the output signals of the two filters will allow a compensation for charge trapping effects to be performed. The simplicity of this method, and the feasibility to implement this solution in analogue integrated circuit with low power dissipation and no switching noise, are appealing motivations to adopt this double filtering stage on the ELBA multi-channel chip.
References [1] F. Lei, A.J. Dean, G.L. Hills, Space Sci. Rev. 82 (1997) 309. [2] E. Caroli, et al., in: M.L. McConnell, J.M. Ryan (Eds.), Proceeding of the Fifth Compton Symposium, American Institute of Physics, CP 510 (2000), p. 809. [3] E. Caroli, et al., Proceedings of the ESA Symposyum on Photon Detectors for Space Instrumentation, ESA SP-356, December 1992, p. 27. [4] J.L. Matteson, et al., Hard X-ray and gamma-ray detector physics, optics, and applications, SPIE 3115 (1997) 160. [5] J.E. Grindlay, et al., EUV, X-ray, and gamma-ray instrumantation for astronomy, SPIE 2518 (1995) 202. [6] F. Lebrun, et al., Gamma-ray and cosmic-ray detectors, techniques, and mission, SPIE 2806 (1996) 258. [7] F.P. Doty, et al., Nucl. Instr. and Meth. A 353 (1994) 356. [8] Z. He, et al., Nucl. Instr. and Meth A 422 (1999) 173. [9] F. Casali, et al., IEEE Trans. Nucl. Sci. NS-39 (1992) 598. [10] W. Dusi, et al., Hard X-Ray, Gamma-Ray and Neutron Detector Physics, SPIE 3768 (1999), p. 88. [11] M. Richter, P. Siffert, Nucl. Instr. and Meth. A 322 (1992) 529.
E. Caroli et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 567–573 [12] J. Lund, et al., IEEE Trans. Nucl. Sci. NS-43 (1995) 1441. [13] E. Caroli, et al., Nucl. Instr. and Meth. A 322 (1992) 639. [14] M. Suffert, et al., Physica 25 (1959) 659.
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[15] M. Sampietro, G. Bertuccio, Electron. Lett. 34 (19) (1998) 1. [16] G. Bertuccio, P. Gallina, M. Sampietro, Electron. Lett. 35 (15) (1999) 1209.
A CdTe position sensitive spectrometer for hard X- and soft g-ray polarimetry E. Carolia,*, J.B. Stephena, W. Dusia, G. Bertucciob, M. Sampietrob, A.J. Birdc, A.J. Deanc, R.M. Curado da Silvad, P. Siffertd, V. Regleroe, W. Yuf, C. Zhangf a Istituto TESRE/CNR,Via Gobetti 101, 40129 Bologna, Italy Dip. di Elettronica ed Informazione, Politecnico, Piazza L. Da Vinci 32, 20133 Milano, Italy c Physics and Astronomy Department, University of Southampton, SO17 1BJ Southampton, UK d Laboratoire PHASE/CNRS, 67037 Strasbourg, France e Dpto. de Astronomia y Astrofisica, University of Valencia, 46100 Burjassot, Spain f Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, China b
Abstract Coded Imager and Polarimeter for High Energy Radiation (CIPHER) is a hard X- and soft g- ray spectroscopic and polarimetric coded mask telescope based on an array of Cadmium telluride microspectrometers. The position sensitive detector (PSD) will be arranged in 4 modules of 32 32 crystals, each of 2 2 mm2 cross-section and 10 mm thickness giving a total active area of about 160 cm2, operating over a wide energy range (B10 keV to 1 MeV). Each PSD module is obtained by aligning 32 linear arrays of micro-detectors each also containing the integrated analog front end electronics on a thin ceramic layer. The CIPHER instrument will be proposed for a balloon experiment, both in order to assess the performance of such an instrumental concept for a small/medium size satellite (or an external ISS-alpha payload) survey mission and to perform an innovative high energy polarisation measurement. Herein we describe the CIPHER position sensitive spectrometer design, together with current development on CdTe detectors array and on a low noise and low power consumption ASIC for the analog front end electronics. Furthermore we present the expected performance in terms of image and spectral quality (spatial and energy resolution) and polarimetric capabilities for such a detector. r 2002 Elsevier Science B.V. All rights reserved. PACS: 95.55.Ka; 95.85.Pw; 95.75.Hi Keywords: CdTe spectrometers; Hard X- and soft g-rays; Polarimetry
1. Introduction Polarisation studies could offer astrophysicists very important information about the emission mechanisms and the physical conditions of cosmic *Corresponding author. Tel.: +39-51-6398-678; fax: +3951-6398-723. E-mail address: [email protected] (E. Caroli).
sources because almost all mechanisms that generate high-energy emission involve strong magnetic fields and lead to the production of polarised photons. Even if polarisation measurements in lower energy bands have been extremely useful, and although telescopes such as the COMPTEL instrument on the Compton Gamma Ray Observatory are theoretically able to perform polarisation measurements the sensitivity is such that no
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 9 2 4 - 6
568
E. Caroli et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 567–573 Table 1 Then main characteristics of CIPHER PSD
(a)
(b) Fig. 1. (a) Shows one of the four CIPHER PSD basic modules, (b) the drawing represents a possible mechanical structure for the four module PSD. The hatched box shows the position of the digital front end electronics.
actual measurements have been performed at energies greater than about 10 keV [1]. In order to fulfil this scientific objective, we briefly describe the design and the current development of a modular position sensitive detector based on CdTe arrays suitable for polarisation measurements on a balloon borne coded mask telescope operative in the range between B10 keV and 1 MeV [2].
2. The position sensitive detector The position sensitive detector (PSD) of CIPHER is made of a matrix of Cadmium Telluride micro-spectrometers. Taking advantage of current CdTe technology, the proposed detector is highly modular. The dimensions of each CdTe crystal are 2 2 10 mm3. These units are used in the
Active pixel size Geometrical pixel size Linear module Matrix module Matrix module number PSD active area PSD geometrical area Detector thickness CdTe mass Linear module support Matrix module container Detector frame PSD overall height
2 2 mm2 2.6 2.1 mm 32 CdTe micro-crystals 32 32 pixels 22 164 cm2 18 15 cm2=270 cm2 10 mm 1 kg Al2O3 (0.3 mm thick) Aluminium Aluminium or carbon fiber 5.5 cm
configuration in which the optical axis is orthogonal to the charge collecting. Using this configuration it is possible to increase the photon detection efficiency at high energies while limiting the spectroscopic performance loss due to charge trapping in the material. These micro-spectrometers are assembled on thin (300 mm) ceramic (Al2O3) plates in linear modules that contain 32 units (with a pitch of 2.1 mm) together with integrated low noise and low power consumption analogue readout electronics as well as the bias circuits (Fig. 1a) The linear modules are packed together at 2.6 mm spacing in a matrix module containing 32 32 CdTe units inside an aluminium case. Below the matrix module, two layers are foreseen containing the hybrid front-end electronics (FEE) with multiplexer and ADCs for the 1024 channels. The matrix module is therefore a complete and independent detector that can be tested and calibrated separately. Finally the CIPHER PSD is formed of 4 matrix modules for a total sensitive area of 164 cm2 (Fig. 1b). The matrix modules are supported by a metallic (Al) or carbon fibre grid. The grid sides provide the mechanical interface for the active veto shield. In Table 1 the main characteristics of the PSD are reported.
3. The detector performance Cadmium telluride is a room temperature semiconductor compound that has been studied
569
E. Caroli et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 567–573 57
4000
Co Collimated Source
3500
Counts
3000 2500 2000 1500 1000 500 0 0
50
100
150
200
250
300
350
Channel 22
Na uncollimated source
10000
Counts
for 20 years as a radiation detector and in the last decade has started to be proposed for space applications in X- and g-ray astronomy [3–6]. CdTe crystals are well suited to the construction of fine spatial resolution position sensitive detectors both using micro-strip (or pixellation) techniques and micro-spectrometer arrays [7]. Even though crystal growing technology has made large advances and has allowed the introduction of a new cadmium telluride compound (CdZnTe), the spectroscopic capabilities of this material is still limited by charge trapping effects. These effects do not allow an increase of distance between the charge collecting electrodes above a few mm, even if, currently, there are promising results in single charge carrier thick CdZnTe detector development [8]. In the traditional use of CdTe detectors the photons are incident on one of the electrodes, thus limiting the absorption thickness and the operative range up to 200–300 keV. Several years ago, the TeSRE group proposed a geometry in which the charge collection field is orthogonal to the optical axis of the detector (PTF: planar transverse field) [9]. In this configuration the charge collection distance is independent from the photon absorption thickness, making this kind of detector suitable for higher energy applications (up to the MeV band). The CdTe micro-crystal configuration proposed for the realisation of the CIPHER PSD has been tested for a long time in the laboratory; a pair of spectra obtained with 2 2 10 mm3 PTF micro-spectrometers are shown in Fig. 2 [10]. These measures has been obtained by selecting events with short rise times: either by collimating a source over a B0.5 mm wide region close to the cathode or by using rise time selection electronics [11]. Using these techniques a typical energy resolution of B5% at 122 keV and 2% at 511 keV are achievable. We are confident that similar results will be obtained implementing an energy compensation based on double shaping technique without significant efficiency loss [12]. In order to evaluate the imaging performance of the CIPHER telescope the knowledge of the point spread function of the PSD is required. The geometrical size of the pixel is 2.1 2.6 mm2, while the PSF is dependent on photon energy and
1000
100 0
100
200
300
400
500
600
700
Channel
Fig. 2. Radiation source spectra obtained in laboratory tests using 2 2 10 mm PTF CdTe spectrometers. These results was obtained with a selection of short rise time signals (we expect similar results by compensating signals with double shaping technique, see Section 5).
incidence angle. The results that we have obtained with numerical simulations indicate that the PSF of the CIPHER PSD does not have a strong dependence on photon energy: the PSF is in general sharper than a Gaussian and is almost independent of energy up to 2 MeV for on-axis sources with a ratio between PSF peak and tails ranging from 120 to 30 (from 0.06 to 1.8 MeV). This energy independence is mainly due to the limited thickness of the PSD, which means that scattered events constitute only a small fraction of the total recorded events (Table 2) [13] and to the high degree of pixellisation that allows an efficient reconstruction of the incidence pixel. The shape of
570
E. Caroli et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 567–573
Table 2 CIPHER total and double event efficiencya E (keV)
Total efficiency
Double events efficiency
20 50 100 200 500 1000
1 1 0.99 0.83 0.41 0.29
0 0.036 0.064 0.170 0.118 0.074
results of numerical simulations indicates that this effect becomes of some importance only for angles greater than 201 and for energies above 500 keV. Therefore we can conclude that the PSF is equal to the pixel geometrical size inside the foreseen CIPHER field of view (B101).
4. Polarimetric capabilities
a
The errors on values are less 2%. (This results have been obtained with Monte Carlo simulation. The double events (events that hits two CdTe pixels) efficiency is directly related to the detector polarimetric capabilities.)
(a)
(c)
(b)
(d)
Fig. 3. The CIPHER PSF at 500 keV: Counts map (A) and reconstructed pixel map (B) for a source on axis; (C) and (D) same maps for an incidence angle of 351. The reconstruction of the incidence pixel in multiple events is obtained with an energy weighted mean.
the PSF is also almost independent (within 10%) from the criterion adopted for the incidence position reconstruction of multiple events (events that hit more than one CdTe element): e.g. mean weighted by energy deposits, or by selection of the pixel with the minimum or maximum energy deposit. In any case, the tails extend at maximum over 4 pixels. Of course the broadening of the spatial response becomes more important as the photon incidence angle increases (Fig. 3). The
The use of a highly pixellated detector is ideal for use as a polarimeter and the CIPHER PSD design, with its high sensitivity, is particularly suited for this purpose. In order to investigate the efficiency of this detector design for polarimetric measurements we have calculated the response of the instrument to a 100% polarized beam of photons in order to obtain the Q polarimetric modulation factor [14]. This is obtained by integrating the formula for the Compton polarimetric differential cross-section: Z ds S ¼ dO dO Z 2 2 r0 e 1 þ e 2 sin2 y cos2 j dO ¼ 2 e where e is the ratio of the Compton scattered energy to that of the incoming photon, r0 is the classical electron radius, y is the angle between the incident and scattered photon direction and j is the azimuthal angle between the scattered photon and the electric vector of the incident photon. The integration takes place over the solid angles defined by the geometry of the detection plane in such a way as to be able to determine the Q-factor defined as N> N8 Q¼ N> þ N8 where N> and N: are the count rates in orthogonal detectors in the X/Y plane. In Fig. 4 we show the Q-factor as a function of photon energy from 100 keV to 1 MeV, showing that this design is very competitive with other proposed instrumentation [1]. In particular this number is related to double-hit event efficiency that for the CIPHER typically ranges from B8% to 16% in the 0.2–1. MeV energy band (Table 2).
E. Caroli et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 567–573
571
5. Current development
Q Factor
0.5
0.4
0.3
100
1000
Energy (keV)
Fig. 4. The Q-factor evaluated for the CIPHER position sensitive detector.
In this section we briefly describe some activities and projects that in particular the TESRE and the Politecnico of Milan groups are performing on the development of this PSD detector in the framework of the CIPHER telescope collaboration. In particular, we have built some linear array prototypes (Fig. 5). These prototype are reduced versions of that proposed for the CIPHER PSD containing only 8 CdTe micro-spectrometers. These prototypes are realised with hybrid techniques, in which only the charge sensitive preamplifier (CSP) is monolithic, while the bias, filter and shaper are made with SMD techniques. The CSP itself is a prototype designed for silicon high
Fig. 5. The pictures show a linear array of 8 CdTe of 2 2 10 mm3 on a 0.5 mm thick ceramic layer. In this prototype each CdTe spectrometer (C) is read out by an integrated CSP (under black coating, A). On the opposite side (B) the SMD filters, bias and shaping circuits for each channel are visible.
Fig. 6. A small matrix of 5 5 CdTe micro-detector that can be used to test the polarimetric capabilities of the CIPHER PSD concept. Each 2 2 10 mm3 crystal is used in PTF configuration and operated at bias up to 150. The CdTe array is mounted on a ceramic disk and is surrounded by the 25 charge preamplifier (Katsura CS507). The aluminium box contains the bias (high and low) distribution board. On the box lateral surfaces are positioned the signal and power connectors as well as the ventilation hole. The energy threshold is around 25 keV.
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energy physics application at the University of Pavia. This linear module will be used in the near future both to asses the real performance of such a position sensitive spectrometer under laboratory conditions and to tune some of the parameters of the low noise and power consumption ASIC that is under development at the Politecnico of Milano. At the same time we are setting up a multichannel test equipment to perform tests on a small CdTe matrix (Fig. 6) to verify the achievable spatial resolution of the detector as function of energy and incidence angle and to assess the polarimetric capability of this kind of position sensitive detector. The requirement for the CIPHER detector to be sensitive down to B10 keV implies the use of a low noise front-end amplifier, at or below 500 electrons r.m.s. for a detector capacitance of about 1 pF. This performance should be reached with minimum power dissipation (o 1 mW/channel) due to the high pixellisation of the detector and to power resource limitations as well as thermal balance problems in space instrumentation. An answer to these requirements will be given by the ELBA (‘‘ELettronica integrata a BAssa potenza’’: low power integrated electronics) project. This low power and low noise characteristic imposed on the front-end electronics has led to an innovative scheme for the preamplifier implemented using BiCMOS technology with the introduction of an active current conveyor stage with a small resistor that acts as a high value feedback resistance [15,16]. The adopted circuit will manage signal over two orders of magnitude amplitude dynamic range with a linearity, inferred from first test measurements, below 1%. The whole amplifier is powered with a single supply voltage of 4 V and dissipates about 550 mW. Extensive experimental tests of the preamplifier and the shaper are presently underway. In Table 3 are shown the main performance and characteristics of the ELBA multichannel chip. Futhermore, the ELBA ASIC design will allow a signal processing suitable for compensating for trapping effects by a double filtering stage for each channel. One filter will operate at a relatively short signal shaping time (from 100 to 250 ns) while the other filter, working in parallel on the same channel, will have a longer
Table 3 ELBA ASICS main characteristics Channel per ASIC Dynamic range Linearity Shaping type Shaping times per channel Peaking time Single power supply Power dissipation (PA+Shaper) Equivalent noise charge Equivalent energy resolution Energy threshold on CdTe
8–16 10–1000 keV o0.3% (Simulation) Unipolar semi-Gaussian 2 (200 ns and 2 ms) 1.5 ms +4 V 590 mW per channel 455e (r.m.s.) 4.5 keV (FWHM) B10 keV (5s)
shaping time (from 1 to 3 ms). The knowledge of the ratio between the amplitudes of the output signals of the two filters will allow a compensation for charge trapping effects to be performed. The simplicity of this method, and the feasibility to implement this solution in analogue integrated circuit with low power dissipation and no switching noise, are appealing motivations to adopt this double filtering stage on the ELBA multi-channel chip.
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