Amonolithic X/gamma-ray detector with an energy range

Transcription

1 1916 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 4, AUGUST 2004 A Pulse Shape Discrimination Gamma-Ray Detector Based on a Silicon Drift Chamber Coupled to a CsI(Tl) Scintillator: Prospects for a 1 kev 1 MeV Monolithic Detector Martino Marisaldi, Claudio Labanti, and Heike Soltau Abstract In this paper, an X- and gamma-ray detector based on a silicon drift detector (SDD) coupled to a CsI(Tl) scintillating crystal is presented. The SDD is operated both as a direct X-ray detector for photons interacting in silicon and as an indirect detector for photons interacting in the scintillator. As interactions in silicon and in CsI yield different amounts of charge per unit energy deposited, discrimination of the place of interaction is necessary to obtain the correct energy. Discrimination of the interaction type is carried out by means of pulse shape discrimination performed with two parallel processing chains with different shaping times. The performance of the detectors are described and discussed. Pulse shape discrimination is possible throughout the detector energy range. Index Terms Gamma-ray detectors, pulse shape discrimination, silicon drift detectors, X-ray detectors. I. INTRODUCTION Amonolithic X/gamma-ray detector with an energy range extended over three orders of magnitude from 1 kev to 1 MeV would be very useful in physics research. Such a detector would be helpful, expecially in high-energy astrophysics: A low-energy detector with good spectroscopic properties would allow, for example, the detection of the Fe lines, and a detector with an energy range extended at least to a few hundreds kev is needed, for example, to discriminate between gamma-ray bursts and other softer X-ray transients. The possibility to combine these two detectors in a single, monolithic instrument would be a great advantage for a space mission, where compactness and limited weight are crucial requirements. We explored the possibility of building such a detector based on a CsI(Tl) scintillating crystal coupled to a silicon drift detector [1]. II. DETECTOR S WORKING PRINCIPLE CsI(Tl) scintillation detectors are widely used for high-energy gamma-ray detection. In fact, as the CsI(Tl) crystal dimension can be up to several tens of centimeters, a high efficiency up to several MeV can be obtained. CsI(Tl), moreover, is well suited Manuscript received February 17, 2004; revised April 26, M. Marisaldi and C. Labanti are with the CNR-IASF sezione di Bologna, Bologna, Italy ( marisaldi@bo.iasf.cnr.it). H. Soltau is with the PNSensor GmbH, München, Germany. Digital Object Identifier /TNS for solid state readout by means of silicon photodetectors, allowing compact design, low-power consumption, and the possibility of making finely pixelated detector arrays. One example of such an instrument is the PICsIT detector on board the IN- TEGRAL satellite, based on an array of 4096 CsI(Tl) crystals coupled to silicon p-i-n photodiodes (PD) [2], [3]. The energy threshold of a CsI(Tl)/PD detector is limited by the electronic noise of the photodiode and by the light collection efficiency and is typically greater than several tens of kev. A gamma-ray detector based on a CsI(Tl) scintillating crystal coupled to a silicon photodetector is, in principle, also a direct X-ray detector, for radiation interacting in silicon. Of course, since a typical photodetector has a significant efficiency only for photons with energy lower than about 30 kev, care must be taken in the detector design to allow radiation to pass through the silicon detector first. Charge pulses either from X-ray interaction in silicon or scintillation light collection are quite different in shape due to the different timing properties and can be discriminated easily. In fact, while the electron-hole pair creation from X-ray interaction in silicon creates a fast signal (about 10-ns rise time), the scintillation light collection is dominated by the fluorescent states de-excitation time [0.68 s (64%) and 3.34 s (36%) for CsI(Tl) at room temperature] and a few s shaping time is needed in this case to avoid significant ballistic deficit. Moreover, interactions in silicon and in CsI yield different amount of charge per unit energy deposited, so discrimination of the place of interaction is necessary to obtain the correct energy. Pulse shape discrimination (PSD) techniques allow discrimination between signals with different timing properties. PSD can be obtained in different ways, both with digital and analog electronics, for example, by means of a constant fraction and a zero-crossing discriminator [4]. In this work, PSD has been obtained simply by sending the signal pulse to two independent amplification chains with different shaping times. The ratio of the pulse heights will be approximately constant for pulses of common shape and will allow discrimination between interactions taking place in silicon or in the scintillator. PSD has been typically applied to the phoswich detectors, where two scintillators with different decay times are sandwiched together and coupled to a single photodetector. Detectors based on a CsI(Tl) crystal coupled to a silicon photodetector operated with PSD have already been described; in [5] and [6], it was proposed as a detector for charged particle discrimination, while in /04$ IEEE

2 MARISALDI et al.: PULSE SHAPE DISCRIMINATION GAMMA-RAY DETECTOR 1917 Fig. 1. Bidimensional spectra obtained with (a) Am and (b) Cs sources. Each image bin is square channels. Channel values have been corrected for ADC offset. Events in silicon in the Cs spectrum are due to the X-ray background and the Ba K X-rays. [7], it was proposed as a X/gamma-ray instrument. However, in all these applications, the silicon photodetector used was a p-i-n PD. Since the energy threshold and the spectroscopic capabilities of such a detector are dominated by the elecronic noise of the photodetector, it was decided to use SDDs instead of traditional PDs, because of their much lower intrinsic noise. Even if SDDs have been developed mainly as direct X-ray detectors [8], they have been also widely tested as photodetectors coupled to CsI(Tl) scintillators [9] [12]. The photodetector s electronic noise is crucial also for detector efficiency considerations, as it will be discussed in Section V. III. EXPERIMENTAL SETUP A 10-mm active area SDD with integrated JFET supplied by PNSensor GmbH was used. The SDD was originally produced with a thin protective aluminum layer on the entrance window, but this layer has been removed by PNSensor GmbH to allow optical contact to the scintillator. The fully depleted active thickness of the device is 300 m. The chip was bonded on a custom made ceramic housing for easy handling and electrical connection. One contact on the backside (the entrance window) and eight contacts on the electronic side are needed for proper device operation. To allow bonding on the electronic side, the ceramic housing was designed with a hole corresponding to the integrated JFET. A CsI(Tl) cylindrical crystal, 10-mm high and 3.6 mm in diameter, was coupled to the detector s entrance window by means of a thin layer of previously cured siliconic resin. The crystal was wrapped with Millipore white diffusive paper and PTFE tape. A PTFE tool was used to keep the crystal in the proper position and to accurately press the crystal onto the transparent siliconic layer. The detector was placed on a custom electronic board for power supply and detector signal preamplification based on an Amptek A250 charge preamplifier. The preamplified signal was sent to two different processing chains each of them made of a Silena 7611 spectroscopy amplifier with quasigaussian shaping and a Silena 7411/N 4096

3 1918 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 4, AUGUST 2004 Fig. 2. Am spectrum masured with the fast acquisition chain (0.5 s shaping time) at room temperature. All events have been considered independently of their slow channel pulse height. Energy resolution at kev is 1.2%. channels ADC. Shaping times were set to 0.5 s on one processing chain (tailored for the detection of X-ray interaction in silicon, hereafter called the fast chain) and 3 s on the other (tailored for the detection of scintillation light, hereafter called the slow chain). The gain has been set to about the same value in the two processing chains. A threshold was set independently for each ADC by means of their low-level discriminator setting. A coincidence signal obtained with a Tennelec TC404A multipurpose coincidence was used to enable the digital conversion only when both discriminators have triggered. A National Instruments DIO-24 digital input/output PCMCIA card was used for ADC handshaking and digital data readout. For each event, the two ADC values were saved on hard disk for subsequent analysis. The detector was irradiated with gamma-ray sources from the electronic side, in such a way that photons pass through the silicon detector first and then, if not absorbed yet, through the scintillator. The hole in the ceramic housing allows gamma rays to reach the silicon detector without attenuation. All measurements have been taken at room temperature. IV. RESULTS A. Detector Performances The detector has been thoroughly tested with Am, Fe, and Cs sources. The spectra obtained can be visualized as bidimensional histograms as shown in Fig. 1, where the two axes are the fast and slow pulse height. As expected, events distribute along two straight lines depending whether interaction takes place in silicon or in CsI. The detector s pulse shape discrimination capabilities are promising and will be discussed in details in Section IV-B. Energy resolution FWHM for X-ray interaction in silicon, calculated by means of the fast chain spectrum, is 1.2% at kev and 5.8% at 5.9 kev. Energy resolution for gamma rays interacting in CsI, calculated by means of the slow chain spectrum, is 6.9% at 662 kev and the corresponding light yield for the gamma-ray detector is 23.4 e kev. The light yield is the charge collected per unit energy deposited in the crystal and depends on the scintillator physical properties, the crystal wrapping, and its optical coupling to the photodetector. As higher light yield values can be found in literature [10], this detector is not optimized due mainly to the optical coupling that is not facilitated by the geometry of the SDD ceramic carrier. A light yield of 26.4 e kev is obtained if calculations are carried out using the kev peak in CsI, and 28.4 e kev using the kev peak. These inhomogeneities are very likely due to the fact that low-energy gamma rays interact within the first few hundreds of micrometers of the CsI crystal, thus photons produced see the SDD entrance window with a much higher solid angle than photons produced deep into the crystals by higher energy gamma rays. Fig. 2 shows an Am spectrum measured with the fast acquisition chain; all events, independently of their slow pulse height, have been included. The Am lines and the Np X-ray fluorescence lines have been used for the X-ray detector calibration and ADC offset calculations, showing an excellent linearity. Some lines not associated with the Np fluorescence X-rays can be easily seen. These lines can be attributed to X-ray fluorescence in CsI as will be discussed in details in Section IV-C. Np fluorescence X-ray line at kev has been used to calculate the SDD electronic noise, assuming that the energy resolution is given by the sum in quadrature of the contributions due to electronic noise and statistical spread. For the fast chain the resulting electronic noise is 41 e rms, and the corresponding energy threshold is 0.8 kev. The slow chain electronic noise is 82 e rms. The corresponding energy thresholds are 1.5 kev for interaction in silicon and 16 kev for interaction in CsI. B. Pulse Shape Discrimination Capabilities For each event, we considered the ratio between the pulse height of the fast and slow processing chains, expressed in channels, corrected for ADC offset fast pulse height slow pulse height for X-rays interacting in silicon (it would be 1 if the gain of the two chains was exactly the same) and for gamma rays interacting in CsI. The distribution can be

4 MARISALDI et al.: PULSE SHAPE DISCRIMINATION GAMMA-RAY DETECTOR 1919 Fig. 3. r distribution for Am events with the slow pulse height between 190 and 490 channels (corresponding to kev for interactions in silicon and kev for interactions in CsI). r is the ratio between fast and slow pulse height. Fig. 5. Figure of merit M versus the slow channel pulse height. defined as the ratio between and the sum of the FWHM and of the distributions [13] (1) Fig. 5 shows the parameter as a function of the slow chain pulse height, for the data reported in Fig. 4. If the distributions are Gaussians, as in this case, it holds the relationship (2) Fig. 4. r distribution centroids for the Si and CsI peaks versus the slow channel pulse height. Each section is 100 channels wide (corresponding to 3.4 kev in silicon and 40 kev in CsI), except for the first two sections that are ten channels wide. Error bars are 61 FWHM. fit with a Gaussian distribution, as shown in Fig. 3, where the distribution for a portion of a Am spectrum with the corresponding gaussian fit is reported. Its standard deviation depends on the pulse height interval it is calculated by. In fact, as pulse height approaches lower values, energy resolution worsens and the width of the distribution widens. Fig. 4 shows the centroid and the width of the distribution for interactions in silicon and in CsI as a function of the slow chain pulse height. Different figures of merit can be defined to establish the PSD capabilities of a detector system. One of these, useful from the theoretical point of view, is where and is the sum in quadrature of the standard deviations of the distributions. For easier visualization, we will consider also the parameter 100% disentanglement of an event can be achieved if, i.e., [14]. This criterion is satisfied for events with the slow pulse height greater than 100 channels, corresponding to 3.6 kev in silicon and 35 kev in CsI. For lower energies, the distributions widen and start overlapping in a significant region, thus 100% disentanglement is not possible anymore. Anyway, PSD is still possible provided. For slow chain pulse height between channel 44 (slow chain energy threshold, corresponding to 16 kev in CsI) and channel 100, it was possible to calculate and, using Cs low-energy Compton tail data, but it was not possible to calculate and because of the lack of events in the corresponding energy range in silicon ( kev) with the radioactive sources used. Anyway, we can assume that at all energies, and is dominated by the electronic noise on the pulse height of the two processing chains. If we consider to be given by the sum in quadrature of the noise contributions of the fast and slow chains, we can estimate at the detector threshold. This is probably a lower limit for parameter, because experimental values are lower than the sum in the quadrature of the noise contributions of the two chains. So, we can assume that PSD is achievable along all the slow chain energy ranges. A higher light yield would lower the energy limit for 100% disentanglement. C. Mixed Interaction Detection and Modeling The Am spectrum reported in Fig. 2 shows at least four peaks next to the keV peak. The energy corresponding to the fast pulse height of these peaks ranges between 28 and 36 kev. These peaks are expected to be X-ray fluorescence lines

5 1920 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 4, AUGUST 2004 Fig. 6. r distribution for a Am spectrum in a pulse height region next to the keV peak in silicon. The peaks due to mixed direct/scintillation interaction are visible. Their r value is clearly different from r =0:92. The small peaks visible in plot (b) are not evident in plot (a) because of the limited resolution of the gray-scale. from cesium and iodine interacting in silicon. In fact, as mentioned above, keV photons typically interact in CsI next to the photodetector, so fluorescence X-rays escaping the scintillating crystal will have a high probability to interact in silicon. Anyway, both the scintillator fluorescence light and the direct interaction in silicon will contribute in coincidence to the overall signal. So, the energy corresponding to these peaks, both in the fast and slow chain spectra, will be a combination of both direct and scintillation signals. The value corresponding to these peaks will lie between and, as shown in Fig. 6. PSD between these peaks and the silicon peaks is good: parameter ranges between 4 and 5.1. Let us introduce a simple model to describe this effect and calculate the correct X-ray energy. Let us consider a gamma ray of energy interacting in the CsI(Tl) crystal in the vicinity of the interface with the photodetector, and let us suppose that a fluorescence X-ray is emitted either by a cesium or iodine atom and absorbed in the active region of the silicon photodetector. If no other energy is lost, the gamma-ray energy is given by where is the energy absorbed in the scintillator and is the energy absorbed in silicon. If we define and, the number of electrons collected by the fast and slow chains, respectively, we can write the ratio slow collected charge as between fast and where and are the electrons collected by the fast chain generated, respectively, by the X-ray interaction in silicon and by the scintillation light, and and are the correspondent electrons collected by the slow chain. For X-ray absorption, either in silicon or in the scintillator only, we can define where can be either Si or CsI, depending on the place of interaction. Moreover, we can write (5) where e kev, given the typical energy ev needed to create an electron-hole pair in silicon at room temperature, and is the system light yield. Defining and substituting (4) and (5) into (3), we obtain (3) (4) (6)

6 MARISALDI et al.: PULSE SHAPE DISCRIMINATION GAMMA-RAY DETECTOR 1921 TABLE I CSI X-RAY FLUORESCENCE PEAKS: PSD PROPERTIES AND ENERGY RECONSTRUCTION To use the value considered up to now, we can write where and are the gain of the fast and slow acquisition chains, respectively, expressed in channels/electrons, obtained by the Am spectra. Thus, (6) becomes The limits of (7) for or approaching 0 are and, respectively, as expected. To evaluate and, we can write where is the energy corresponding to the peak in the slow chain. Equations (7) and (8) can be rearranged in order to obtain The calculated values for and the corresponding transition energies in Cs and I are shown in Table I. The error on is about 4%, and it has been obtained by propagating errors on parameters in (9). A good consistency of the calculated values with cesium and iodine KL and KM transition energies [15] has been found. On the contrary, was found to be at least 20% higher than the expected value (59.54 kev- ). We believe this is because of the light yield shift next to the photodetector s entrance window, as we have already discussed in Section IV-A. In fact, (9) for includes the term related to the light yield. In calculations, we used the value 26.4 e kev obtained with the keV peak in CsI, but interactions originating the observed peaks are likely to occur nearer the entrance window, thus with a higher light yield. With a more accurate estimation for, it is possible to evaluate the correct incoming photon energy. The model discussed above shows that mixed interactions can be easily detected. It should be remarked that, with photon energies of 60 kev or higher, the probability of a mixed interaction is much lower than that of a full interaction in the scintillator. This is evident from Fig. 2, where the peak corresponding to the keV photons interacting in CsI is much more populated than the CsI X-ray fluorescence peaks. The fate of the mixed events, whether they have to be corrected or rejected, would depend on the detector s specific application. (7) (8) (9) Fig. 7. Expected detector efficiency for on-axis photons. Solid line: Efficiency for interactions in silicon. Dashed line: Efficiency for interactions in CsI. The energy thresholds of the slow chain for both types of interactions are shown. V. DETECTOR EFFICIENCY AND NOISE CONSIDERATIONS In Fig. 7, the expected efficiency of the detector for on-axis photons is shown. The energy thresholds of the slow chain for interaction in silicon 1.5 kev and in CsI 16 kev are also shown. The efficiency for the SDD is limited at low energy by the dead layer thickness due to the integrated electronics and polarization rings. has been calculated computing the absorption coefficients for all the dead layers involved (aluminum, silicon oxide, silicon nitride, and nondepleted silicon) by means of the photon cross sections provided by [16]. The K absorption edges for aluminum and silicon are clearly evident in Fig. 7. The efficiency for the CsI crystal coupled to the SDD is limited at low energy by photon absorption in the SDD itself, and at high energy by the crystal thickness. About 25% efficiency at 1 MeV is obtained with a 1-cm thick crystal. A 500- m thick dead layer between SDD and the crystal has been considered in calculations to account for the optical coupling material. The detector total efficiency at energy will be for and for. So, for energies between about 16 and 200 kev. For energies between about 10 and 16 kev, there is an efficiency gap due to the decrease of. This efficiency gap could be further reduced lowering the SDD electronic noise, for example, by moderate cooling of the device. The SDD used was not developed for this application. An optimized design of the SDD electronic side in order to reduce the dead layers would result in higher efficiency in silicon at low energy. SDDs production process is now optimized by antireflecting coating at the entrance window, not present on the devices used in this work. The use of these enhanced devices would allow a light yield improvement. The use of a low-noise photodetector such as SDD is clearly essential for this application. In fact, a higher electronic noise would shift and consequently at higher energies, thus widening the efficiency gap for energies below. Moreover, the and distributions would have higher FWHM, thus reducing the PSD figure of merit. The PSD capabilities of the detector would be degraded and the energy

7 1922 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 4, AUGUST 2004 threshold for 100% events disentanglement would be shifted at higher values. If we consider a traditional p-i-n photodiode with, for example, the electronic noise five times higher than that of SDDs and would be 7.5 and 80 kev, respectively. This would lead to a deep efficiency gap between 20 and 80 kev. Moreover, the parameter would be reduced by about a factor five, so the energy limits for 100% disentanglement would be 20 kev in silicon and 200 kev in CsI. VI. CONCLUSION A novel X/gamma-ray detector based on a silicon drift detector coupled to a CsI(Tl) scintillator operated by means of pulse shape discrimination is presented. The SDD works both as a direct X-ray detector for photons interacting in silicon and as a photodetector for the scintillation light generated by photons interacting in the CsI crystal. Pulse shape discrimination is carried out by means of the comparison between signals provided by two parallel processing chains with different shaping time. The device has been succesfully built and tested. The energy threshold is 1.5 kev for the direct X-ray detector and 16 kev for the scintillation detector. Energy resolution FWHM for X-ray interaction in silicon is 1.2% at kev and 5.8% at 5.9 kev, while energy resolution for gamma rays interacting in CsI is 6.9% at 662 kev, with a corresponding light yield of 23.4 e kev. A nonlinearity in the light yield at low energy has been observed. It is very likely caused by light collection enhancement for photons interacting in the crystal next to the photodetector, and could be evaluated with, for example, a tunable X-ray source. Pulse shape discrimination is possible along all the detector energy range, even if 100% accuracy can be achieved for energies greater than 3.6 kev in silicon and 35 kev in the scintillator. Mixed interactions, where a photon interacts in CsI and a fluorescence X-ray is absorbed in silicon, can be easily detected. A simple model based on experimental parameters can be used to calculate the fluorescence X-ray energy with a few percent accuracy. Main improvements in the detector performance can be achieved by reducing the SDD electronic noise and increasing the scintillator light yield. A reduction in the SDD electronic noise, for example, by moderate cooling of the device, would lead to improvements in threshold, energy resolution and efficiency. Anyway, due to the variations with temperature in CsI(Tl) scintillation yield and fluorescence decay times, a tradeoff must be performed to find out the operating temperature for optimal performance. A higher light yield would improve the spectroscopic capabilities of the scintillation detector and would lower the threshold for 100% accurate pulse shape discrimination. The proposed detector combines a low-energy X-ray detector with good spectroscopic capabilities, with a gamma-ray detector with significant efficiency up to 1 MeV in a single, compact device. Such a detector would be of great interest, expecially for X and gamma-ray astrophysics where space and weight optimization is a strong requirement. ACKNOWLEDGMENT The authors would like to thank E. Rossi and A. Mauri at CNR-IASF, Bologna, for skillful electronics prototyping and help with crystal preparation. The authors would also like to thank P. Lechner at PNSensor GmbH, A. Longoni, and C. Fiorini at Politecnico Di Milano, and F. Perotti at CNR-IASF, Milano. REFERENCES [1] E. Gatti and P. Rehak, Semiconductor drift chamber An application of a novel charge transport scheme, Nucl. Instrum. Meth. A, vol. 225, pp , [2] C. Labanti, G. Di Cocco, G. Malaguti, J. Stephen, E. Rossi, F. Schiavone, A. Traci, G. Ferro, S. Ferriani, A. Mauri, and D. 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