Pixels GaAs Detectors for Digital Radiography. M.E. Fantacci. and. Abstract

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1 Pixels GaAs Detectors for Digital Radiography M.E. Fantacci Dipartimento di Fisica dell'universita and Sezione I.N.F.N., Pisa, Italy and European Laboratory for Particle Physics (CERN), Geneve, Switzerland Abstract Gallium Arsenide is a semiconductor in principle well suited to perform Digital Radiography in clinical conditions, because its high Z-value permits an high detection eciency thus allowing a dose reduction to the patient or an improvement of the imaging performances. If the application is mammography, the mean photon energy is about 20 Kev and the choice of an appropriate material for the detector is essential. A pixel structure is suggested because of its low signal-to-noise ratio and its acceptable spatial resolution. However, in the semi-insulating GaAs, are usually present defect centers, which can seriously aect its detection properties. In this paper are shown the performances of various GaAs crystals and their matching with the imaging requirements. A preliminary image obtained with a semi-insulating GaAs pixels detector ia also presented. 1. Introduction To perform a digital radiography in clinical conditions with good imaging performances there are many parameters to be considered in the detection system (cfr. contributed paper of E. Bertolucci at this workshop). In particular, there are many requirements related to the front-end and the read-out electronics, and many related specically to the detector properties. The ultimate goal of this project is a system able to perform mammography. This implies that the incoming X-ray ux is peaked at 20 kev, with a gaussian spread, and a minimum and maximum energy cut of 10 kev and 30 kev, respectively. The most relevant physical X-rays absorption processes inside the irradiated organ (breast in this case) take place at energies about 20 kev. In order to nd the best material for the detector, experimental measurements and Monte Carlo simulations have been made on GaAs materials from dierent factories, of dierent thickness and equipped with dierent electric contacts [1, 2]. The properties under investigation were the detection eciency (), the charge collection eciency (cce), the energy resolution (ER) and the spatial resolution. Regarding the detection, a good detector for digital radiography has to be highly ecient, in order to reduce the dose to the patient or, with the same dose, to decrease the lower limit of the minimum detectable contrast. The comparison term is, for the energy of 20 kev typical in mammography, the value of 57% of the best lm-screen system now available. A high cce is essential to detect the signals coming from low-energy X-rays. The minimum cce for a GaAs detector able to perform digital radiography is 80%. This value has been calculated taking into account the energy gap of GaAs and the unavoidable electronic noise. The threshold for the front-end electronics has to be set at a minimum of 3 standard deviations from the value of the noise. It is essential to have a good signal well separated from the noise for a correct setting of threshold, which has to be common for various channels with unavoidable disuniformities, and 1

2 so a good ER is needed, about 10%. For the same reason, to avoid signal disuniformities, the cce has to be as constant as possible with respect to the applied bias. Acceptable (cfr. contributed paper of E. Bertolucci at this workshop) spatial resolutions can be obtained with pixels dimensions ranging between 100 and 500 m. In this paper a comparison, of these detection properties, between dierent GaAs detectors is reported, and a preliminary image obtained with the chosen detector is presented. 2. Detectors Description The detectors considered were a LPE (Liquid Phase Epitaxy) and 7 LEC (Liquid Encapsulated Czochralski) detectors. The LPE detector has been made by the Australian Nuclear Science & Technology Organization (ANSTO) by depositing an epitaxial layer 120 m thick of high purity n-type on an n + - type 500 m thick. The epitaxial surface has then been equipped with a Schottky contact consisting in a circular gold contact of 1:6 mm diameter [3]. For each LEC detector, in table 1 are reported the rm where the material has been fabricated, the rm or institution where the contacts have been evaporated, the thickness and the diameter of the Schottky contact. The detectors # 2, # 4 and # 5 present a Table 1: Fabrication and geometrical characteristics of our detectors. + The rear surface has been polished. ++ Side length of the square pixel. # Mat. rm Cont. rm Thick. (m) (mm) 1 AXT ALENIA MCP Un. Glasgow MCP Un. Glasgow SUMITOMO ALENIA SUMITOMO ALENIA NIPPON M. Un. Lecce NIPPON M. Un. Lecce 70 0:2 ++ double Schottky contact, all others have an ohmic contact on the back surface. The characteristics of the contacts are described in the contributed paper of A. Cola at this conference. 3. Choice of the Detector The signals from the irradiated detectors were collected by a charge-sensitive preamplier (OR- TEC 142A), a shaper amplier (ORTEC 673) with a shaping time of 1:0 s and a multichannel analyser. All the measurements were performed at room temperature. To measure the various crystals have been irradiated with 22, 60, 88 and 122 kev photons and the results have been compared with a 300 m thick Silicon crystal. The results, shown in the contributed paper of E. Bertolucci at this conference, are in very good agreement with the predictions of a Monte Carlo simulation based on the EGS4 code [4]. For each energy, does not depend on the material and contacts characteristics and, in the diagnostic energy range, GaAs works better than Silicon: at 20 kev photon energy, a 20 m thick GaAs detector exhibits a of 37:2%, while 300 m thick Silicon detector has, at the same energy, a of 24:2%. This advantage of GaAs over Silicon is even more important when higher photon energies are required. Thus the use of GaAs allows to perform digital radiography at an energy greater than 20 kev (application to bone radiography) or to perform a digital mammography with an important reduction of the dose to the patient [5]. A cce complete and independent with respect to the bias voltage has been obtained only with the LPE detector [6]. In fact, epitaxially grown crystals (LPE) are, in principle, relatively exempt from defects and trapping centers and so all the charges generated in the active volume of the detector can be collected; moreover the trapping centers can be detected and taken into account in terms of density, cross sections and activation energies. In a previous work [6] these quantities have been fully measured and used to build a model, including the parametrization of the electric eld inside the crystal, which is in very good agreement with the measured detection performances. On the other hand, the obtained with the same detector is still too low with respect to 2

3 the lm-screen systems, due to the active thickness available which is lower than the physical thickness of the crystal. To grow hundreds of microns thick LPE crystals is still a slow, expensive and delicate process and, moreover, even for the thicker crystals such as the ANSTO (120 m), the active volume is low (about 20 m in our case) because the low resistivity of the material causes the breakdown before reaching the full depletion. The LEC crystals considered are thicker (see table 1) and presented the possibility of a fully depletion, so they are more ecient in the detection of photons. In previous works [7, 8] has been shown that the behaviour of the cce and the ER as a function of bias voltage was similar for various photon energies (22 and 88 kev from 109 Cd, 60 kev from 241 Am and 122 kev from 57 Co). This fact indicates that these detection properties are intrinsic characteristics of each detector. This behaviour is conrmed also by detector # 1 (see gures 1 and 2). In table 2 are reported the maximum bias operative voltage, the cce and the ER at the maximum bias operative voltage for each detector with 60 kev photons. The cce versus applied voltage increases with voltage and the shapes (see gure 1) are similar for all the detectors, but the values are very dierent, as we can see in table 2 for the maximum value of the applied voltage. On the contrary, the ER for each detector is almost constant varying the applied voltage. Even in this case the values are very dierent for the various detectors as we can see in in table 2. Two of these crystals (detectors n.2 and n.7) exhibit a charge collection eciency greater than 80% and an energy resolution (/E) better than 10%. In particular, the detector n.7 has been chosen to realize a pixel structure in order to evaluate his imaging capability. 5. Imaging Performances In order to investigate the imaging capability of a GaAs pixel detector, a matrix of 36 square pixels was deposited on the 70 m thick substrate (detector n.7 in the precedent section). Each pixel had an area of 200x200 m 2 with 20 m spacing between adjacent pixels. The measurements [9] of the cce and of the ER performed at 60 kev photon energy at various bias voltages are reported in gures 3 and 4, respectively. At the maximum bias voltage (80 V) the detector shows a cce of 87:6 4:8% and an ER of 5:5 0:3%. The gure 5 shows the relative position of a 100 m thick wolframium slab with respect to the detector. A theoretical SCR th = 50:0% and SN R th = 4:3 result from the denitions [10]. This system was successful in recording (at 60 kev photon energy) the image of the wolframium slab (gure 6) with SCR exp = 62:8 16:3% and SN R exp = 5:5 1:7, and, most important, the cce uniformity dened as: = 100 h 1? (cce max? cce min ) cce max among the dierent pixels is better than 88%, which ensures that a comfortable common threshold setting can be adopted without aecting the uniformity of the measurement. 7. Discussion and Conclusions The LPE detector has a high and constant cce and a very good ER and the MonteCarlo simulation, i (1) Table 2: Charge collection eciency (cce) and energy resolution (ER) at 60 kev photon energy. # V bias cce (%) ER (%) :7 8:8 20:2 4: :9 6:1 7:5 0: :4 11:5 32:7 10: :5 15:0 24:8 6: :1 14:7 23:4 5: :1 14:9 38:0 10: :6 4:8 5:5 0:3 3

4 Figure 1: Charge collection eciency at various energies as a function of bias voltage for detector # 1 [2]. Figure 2: Energy resolution at various energies as a function of bias voltage for detector # 1 [2]. 4

5 Figure 3: The charge collection eciency as a function of the bias voltage [9]. Figure 4: The energy resolution as a function of the bias voltage [9]. 5

6 Figure 5: Top and side view of experimental apparatus to record the image of a wolframium slab using our GaAs pixels dectector [9]. Figure 6: Image of a wolframium slab thick 100 m collected (at 60 kev) by a matrix of 6 x 6 pixels, each 200 x 200 m 2 of GaAs with an active depth of 70 m. In the right side the grayscale is shown [9]. 6

7 which takes into account the electrical measurements, reproduces very well the results obtained by the irradiation measurements. However, the low active thickness make impossible an immediate application of such a detector in Digital Radiography, because the although higher than the 300 m thick Silicon detector is still lower than the best lm-screen system now available. For the LEC detectors, the values of cce and ER are not very dependent on the material and its processing; on the contrary, they depend strongly on the thickness of the crystal as we can see comparing detectors from the same manufacturer such as the couples # 2-# 3 and # 6-# 7 even though in the latter case the big dierence of architecture can play a role. A model of the possible inuence of the thickness, relating it to the deep levels action in the trapping mechanisms is also described in [11, 12]. LEC detectors 70?80 m thick present satisfactory values for, cce and ER. The high value of charge collection eciency uniformity measured on a 36 pixels matrix obtained with the 70 m thick material and the image acquired with the same structure make this detector very promising for Digital Radiography. 8. Acknowledgements Thanks are due to Prof. F. Nava (University of Modena) and Dr. S. D'Auria (University of Glasgow) for making some detectors available to us. The author would like to thank Mr. C. Magazzu and A. Profeti for their skillful help. References [1] W. Bencivelli et al.,mat. Res. Soc. Symp., Proc. 302 (1993) 381. [2] E. Bertolucci et al., "Electrical characterization and detection performances of various semiinsulating GaAs detectors for low energy Gamma-rays", IEEE Transactions on Nuclear Science in press. [3] D. Alexiev et al., Nucl. Instr. and Meth. A 317 (1992) 111. [4] W. Bencivelli et al., Nucl. Instr. and Meth. A 310 (1991) 210. [5] R. Bertin et al., Nucl. Instr. and Meth. A 294 (1990) 211. [6] W. Bencivelli et al., Nucl. Instr. and Meth. A 346 (1994) 372. [7] W. Bencivelli et al., Nucl. Instr. and Meth. A 338 (1994) 549. [8] W. Bencivelli et al., Nucl. Instr. and Meth. A 355 (1995) 425. [9] E. Bertolucci et al., "X-Ray Imaging using a Pixel GaAs detector", Nucl. Instr. and Meth. in press. [10] D.R. Dance, The Physics of Medical Imaging (S. Webb Editor, 1990) p.27. [11] F. Nava et al., Nucl. Instr. and Meth. A 349 (1994) 156. [12] G.F. Knoll et al., Ionizing Radiation, M.R.S. 302 (1993) 3. 7