On the development of compound semi-conductor thallium bromide detectors for astrophysics

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1 On the development of compound semi-conductor thallium bromide detectors for astrophysics A. Owens *1, M. Bavdaz 1, I. Lisjutin 2, A. Peacock 1, S. Zatoloka 2 1 Astrophysics Division, ESA/ESTEC, Postbus 299, 2200AG Noordwijk, Netherlands 2 Baltic Scientific Instruments, 26 Ganibu dambis, PO Box 33, Riga LV-1005, Latvia We discuss the detector requirements for future X-ray astrophysics missions and present preliminary results from our compound semiconductor program designed to produce X-ray detectors with high spatial and spectral resolution across the energy range 1 kev to 200 kev. Several prototype detectors have been fabricated from monocrystalline TlBr and tested at hard X-ray wavelengths in our laboratories and at the ESRF synchrotron research facility. Energy resolutions of 1.6 kev (fwhm) at 5.9 kev and 2.6 kev (fwhm) at 26 kev have been achieved, although we find that performance is highly variable due to polarisation effects. The resolution function is dominated by high leakage current at all energies. From pulse height measurements of Am 241 as a function of detector bias, we derive the electron mobility-lifetime product at -2 o C to be (2.9±0.2) 10-4 cm 2 V -1. This is about an order of magnitude higher than previously reported values. Keywords: Compound semiconductors, TlBr, X-ray astronomy PACS: 07.85N, 29.40W 1. Introduction In astrophysics, the hard X-ray regime between 10 kev and 200 kev is relatively unexplored even though it includes the important transition region between thermal and non-thermal emission processes predicted to occur throughout the Galaxy. Likewise in planetary physics, Solar X-ray fluorescence imaging of planets and small solar system bodies can yield scientifically important composition maps. At present there is a paucity of such data. Information on non-thermal processes has been largely gleaned from gamma-ray measurements, which are fraught with difficulties due to poor detection efficiencies, high backgrounds and poor directional discrimination. Hard X-ray measurements can provide a direct channel with which to probe a wide variety of non-thermal processes, but to date such measurements have been limited by the lack of efficient detectors and focusing or concentrating optics. Recent developments in microchannel plate optics [1] offer a possible solution to the later, while developments in compound semi-conductors show that such materials offer a * Corresponding author. aowens@astro.estec.esa.nl, tel , fax

2 viable alternative to Si or Ge. In addition, materials drawn from group III-VI compounds have high enough atomic numbers to ensure good detection efficiencies above 10 kev. They also have an additional advantage in that their band-gaps are sufficiently high so they do not require cryogenic cooling but low enough that subkev spectral resolution can be achieved at hard X-ray energies. For higher energies, thallium bromide is a particularly attractive material because of its wide bandgap (2.678 ev), high atomic number (Tl=81, Br=35) and high density (7.5 gm cm -3 ) and hence good stopping power for hard X- and gamma rays [2]. In this paper we present preliminary results from several prototype TlBr monolithic detectors which have been tested in our laboratories and at the ESRF synchrotron research facility. Fig. 1. Schematic of the prototype detector design. 2. Detector fabrication TlBr has a CsCl-type simple cubic crystal structure. Its physical properties are amenable to easy and rapid purification and standard growth techniques. It melts congruently at 480 o C, allowing good quality crystals to be grown directly from melt. The detectors studied here were cut from a thermally grown monocrystal. The boule 2

3 was sawn into several 1 and 2 mm thick slices and the detectors diced from the wafers. The typical dimensions were mm 3. The samples were mechanically lapped followed by mechanical and chemical polishing. Aquadag contacts were then applied to the polished surfaces and the device connected to the outside world by a pressure-contacted copper rod on the bottom surface and a beryllium-bronze leaf spring on the top. A schematic view of a detector is shown in Fig.1. Figure 2. Composite response of the detector 2.7 to an Am 241 and Fe 55 radioactive sources. The detector numbering convention is defined as follows. Detector 2.7 is the seventh detector diced from wafer two. 3. Experimental After initial stability and noise tests of 4 devices, three were packaged into detectors. They were mounted in ceramic holders and glued to two-stage Peltier coolers capable of cooling the devices to ~ -35 o C. The analog chain consisted of a charge sensitive preamplifier (with a FET 2N4416 input FET and feedback resistor, R f = 1 GΩ) used in conjunction with an Ortec 671 spectroscopy amplifier. From the I-V characteristics, their resistivities were found to be in the range (6-10) Ω cm. At nominal biases of ~ 130V, the average recorded leakage currents were ~10 na at room temperature and 1 na at -2 o C. Two devices (detectors 2.7 and 3.1) were used for testing in our laboratories and a third (detector 2.3) at the ESRF synchrotron. 3

4 Fig. 3. The linearity of detector 2.7 measured over the energy range 6-60 kev. The lower panel shows the residuals of a best-fit linear regression to the data. Fig. 4. The measured fwhm energy resolution ( E) of detector 2.7 under full uniform illumination. The solid line shows the best fit resolution function to the combined data set. Here e is the electronic noise of the system measured with a pulser. 4

5 The detector numbering convention is defined as follows - detector 2.3 means it was the third detector diced from wafer two. Initial tests showed that biasing the detector to preferentially collect holes gave quantitatively better spectral resolution than collecting electrons. Furthermore, there was no optimum operating temperature when collecting electrons, in agreement with results of Shoji et al. [3]. However, an optimum operating temperature was -2 o C was found when collecting holes. The detectors measured in our laboratories were biased to collect holes. A shaping time of 3 µs was used for most measurements. 3.1 Laboratory measurements Figure 2 shows a composite response of detector 2.7 to Fe 55 and Am 241 radioactive sources. The fwhm energy resolution was measured to be 1.6 kev at 5.9 kev and is dominated by electronic noise ( e=1.6 kev fwhm). (Detector 3.1 give a slightly worse resolution of 1.9 kev fwhm). The noise threshold is 3 kev. The measured energy resolutions for the Np Lα X-ray at 13.9 kev and the nuclear line at 26.3 kev were 1.8 kev and 2.6 kev, respectively. In Figure 3 we show the linearity curve of the detector over the energy range 6-60 kev. From a best-fit straight line to the peak channel versus energy data, we determine the average non-linearity to be 0.6%. The lower panel shows the residuals, i.e., (measured energy - energy)/ energy 100%. In Figure 4, we show the energy dependent fwhm energy resolution of the detector. For comparison the electronic noise of the system is also shown. The mobility-lifetime product for electrons (µτ) e was determined the Np Lα, Lβ and Lγ X-ray peaks of Am 241 at 13.9, 17.5 and 21.0 kev and the nuclear line at kev. The photopeak pulse heights were measured as a function of detector bias and fit to a single carrier Hecht equation [4] given by, H / H o = ((µτ) e V / d 2 ){1 exp( d 2 /(µτ) e V )} (1) where H is the measured pulse height, H o is the pulse height that would be obtained if the detector was 100% efficient, V is the bias potential applied to the detector and d is the thickness. Assuming that, d 2 /(µτ) e V << 1, then H/H o 1/V and (µτ) e is related to the slope of H/Ho versus 1/V. A best-fit straight line yielded a χ 2 of 9 for 10 degrees of freedom. The derived (µτ) e product was (2.9±0.2) 10-4 cm 2 V -1 at -2 o C which is about an order of magnitude higher than previously reported values [5,6]. 3.2 Synchrotron measurements The third detector (number 2.3) was tested on the open bending magnet (BM5) high energy beam line at the European Synchrotron Research Facility (ESRF). The beamline uses a double Si[111] crystal monochromator to produce highly 5

6 monochromatic X-ray beams tuneable over the energy range 7-35 kev with an intrinsic energy resolution of ~15 ev. The detector was mounted on a 2-axis X-Y table capable of positioning the detector to a precision of ~ 1 µm. For the majority of measurements, the beam was normally incident on the centre of the detector and had a typical spot size of µm 2. Fig. 5. ESRF room temperature response of detector 2.3 to incident radiation of energy 25 kev. In Figure 5 we show the detectors response to 25 kev incident X-rays from which it is clear that the detector is noisier that the one used in the laboratory measurements. The temperature of the detector was 25 o C. The peaks at 75 kev and 100 kev are the 3rd and 4th harmonics (the 2nd harmonic is forbidden for Si[111] reflections). From the figure, we conclude that the detector response is dominated by leakage current since the widths of the peaks are independent of energy. The measured fwhm energy resolution at 25 kev is (7.7±0.2) kev. No further energies were measured due to a detector malfunction. The detector was raster scanned in the plane perpendicular to the beam axis to map the spatial uniformity of the count rate response. Figure 6 shows the count rate profile at 25 kev from which we see that the spatial response is highly non-uniform with the bulk on the charge collection occurring close to the contact. We interpret the 6

7 horizontal depression running across the count rate profile as X-ray absorption in the contact wire, since its width is compatible to the wire thickness of 0.3 mm. Fig. 6. Spatial response of detector 2.3 measured at the ESRF. The incident beam energy is 25 kev and the spatial resolution is 150 µm. The beam size is µm Discussion and conclusions The results obtained from our prototype detectors are encouraging yielding performances similar to other well established compound semiconductor technologies. The derived value of (µτ) e is about a factor of 10 higher than previously reported [5,6] and as good as those reported for established materials such CdTe and HgI 2 [7]. Although the transport properties of electrons and holes in TlBr are similar [6] (and thus a relatively large number of carriers are detected per X-ray event), shot noise caused by high leakage currents currently dominates the energy resolution function. This is believed to be a direct consequence of the relative softness of TlBr compared to other materials (Knoop hardness=12 kg mm -2 ). Any mechanical treatment (i.e., cutting, lapping and polishing) generates a high concentration of intrinsic structural defects by local plastic deformation. The depth of these defects can be surprisingly large (up to 1 mm). We are now experimenting with new handling and surface treatment procedures. Lastly, based on the non-uniform spatial response we are now developing other contacting technologies although for basic material 7

8 investigations, aquadag contacting is still a quick, inexpensive and convenient alternative. References [1] M. Beijerbergen, M. Bavdaz, A. Peacock, E. Tomaselli, G. Fraser, A. Brunton, E. Flyckt, M. Krumrey and A. Souvorov, Proceedings of the SPIE, 3765 (1999) 452. [2] K. Shah, J. Lund, F. Olschner, L. Moy and M. Squillante, IEEE Trans. Nucl. Sci., 36 (1989) 199. [3] T. Shoji, K. Hitomi, O. Muroi, T. Suehiro, and Y. Hiratate, IEEE Trans. Nucl. Sci., in press. [4] K. Hecht, Z. Physik, 77 (1932) 235. [5] F. Olscher, M. Toledo-Quinones, K. Shah, and J. Lund, IEEE trans. Nucl. Sci., 37 (1990) [6] K. Hitomi, T. Murayama, T. Shoji, T. Suehiro, and Y. Hiratate, Nucl. Instr. and Meth., A428 (1999) 372. [7] D. McGregor, in Semiconductors for room temperature nuclear detection applications, eds. T. Schlesinger and R. Jones, Academic press, New York (1995)