951 NEW X-RAY DETECTORS FOR XRF ANALYSIS Jan S. Iwanczyk & Bradley E. Patt Photon Imaging, Inc., Northridge, CA 91324 Abstract The use of miniaturized XRF instrumentation for in-vivo applications imposes specific requirements on the choice of detector technologies that can be used, and on the design and construction of the instrumentation. One critical issue is the temperature of operation of the detector. Substantial cooling cannot be tolerated because of the difficulties of extracting the heat from a small probe. Another issue is the bias voltage of the detector, which impacts the safety design. Issues related to complexity of the device, ease of use, and options for sterilization between uses are also important considerations impacting the choice of detector technology. In this paper we present recent advances in detector technologies which open up new possibilities for in-vivo applications of XRF. In particular we discuss the characteristics of HgI,, CdTe, Si (pi-n), and Silicon Drift Detectors (SDD s) including their basic material properties, suitable detector structures, and state-of-the-art performance. The usefulness of the various technologies for in-vivo X-ray spectroscopy applications is discussed. Introduction There has been a continuous effort to develop non-cryogenic energy-dispersive compound semiconductor x-ray detectors for XRF applications. With advances in material characteristics and detector design, excellent results have been achieved with HgI,, CdZnTe, and CdTe (p-i-n) x-ray detectors. Resolution of better than 200 ev FWHM for HgI, detectors (at 0 C) [l] and better than 250 ev FWHM for both CdZnTe and CdTe(p-i-n) detectors (at -40 C) [2,3] have been measured at 5.9 kev. The above results were obtained with the use of a miniaturized, low-power thermoelectric coolers and dry nitrogen atmosphere. The active area of these detectors was about 10 mm2. With the availability of high quality, high resistivity silicon and better surface passivation techniques, encouraging results have also been obtained recently with Si (p-i-n) detectors. We have obtained energy resolution better than 220 ev FWHM at 5.9 kev with a device operating at about -40 C [3]. The active thickness of the device was about 300 pm. Thus the device has limited efficiency (less than 63% absorption) above 13 kev. In addition, the small thickness of the Si (p-i-n) structures limits their active area to only 6-7 mm2. Larger active areas would have too high detector capacitance resulting in increased electronic noise. This limitation can be overcome by using a novel structure, namely the silicon drift detector (SDD) structure [4]. This structure has a small charge-collection anode and cathodes that are designed to focus the charge (electrons) to the small anode. The device capacitance (anode capacitance) is minimized and is
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952 independent of the detector size because the collection anode can be kept very small. SDD structures with an energy resolution of 145 ev (FWHM) at 5.9 kev have been shown [5] at an operating temperature of -20 C. However, these devices have a limited active area of 3 mm2. This paper will describe our efforts to obtain large SDD structures (close to 1 cm ) for X-ray spectroscopy applications. High Z Compound Semiconductors High atomic number (high-z) and wide band-gap compound semiconductors have been found to be an attractive choice for room-temperature x-ray spectrometers. The broad band-gap of these materials allows for a low detector leakage current and with a construction designed to keep detector capacitance below 1 pf, the detector electronic noise can be very low. Because of the high Z of the detector material, x-rays are absorbed very close to the entrance electrode. This feature helps to create a single-charge-collection situation which is very important for compound materials such as HgI,, CdTe and CdZnTe where electrons are much more mobile than holes. When all x-ray interactions take place near the negatively biased electrode, the generated holes travel only a minuscule distances, while the electrons must cross the detector s entire active thickness. In this case, the contribution of the holes to the induced pulse amplitude may be neglected and essentially only the electrons contribute to the spectral response. Over the past several years, a combination of low leakage currents and relatively good electron collection have resulted in energy resolution in the vicinity of 200 ev (FWHM) at 5.9 kev for HgI,, CdTe and CdZnTe compound detectors. The construction of the spectrometers and experimental details are given elsewhere [2,3]. Silicon Drift Detector (SDD) Basic Operation The basic configuration of the silicon drift x-ray detector is shown in Fig. 1. The p+ electrodes on the front and the back side are reverse-biased, so that the bulk of the detector is fully depleted, producing a parabolic-shaped potential valley within the detector from the front to the back side. The bottom of the drift valley corresponds to the minimum potential energy for electrons, and is where the electrons are transported. By applying a radial potential gradient across the back surface of the detector (with decreasing potential from V, to V,), the drift valley can be forced to tilt along the detector toward the n+ collection anode as shown in Fig. 1. The detector structure thus focuses the electron collection to the small anode with the induced signal occurring predominantly between the anode and the adjacent focusing electrode. In the type of structure depicted in Fig. 1, the anode capacitance is minimized (and is independent on the detector size) because the collection anode can be very small (80 pm diameter). Our approach, described below, has several new features incorporated into the design. These include: p+/n+ concentric rings on the back side, and equipotential bias on the front side.
953 -- 1 cm -------J I I., I v2 p+ enpance electrode,- / I 1 L Figure 1: \ I p electrode HV termination guard node Cross section of cylindrical drift detector (Basic concept) The investigation of the various contacting configurations, biasing voltages, dimensions of the structure, and resulting electric field distributions has been accomplished by computer simulation using 2D simulation tools such as SILVACO and MEDIC1 physical device simulator packages VI. The theoretical noise limit of the SDD was determined from calculations discussed in [4] to be about 8 em RMS for the optimal case with integrated FET and - 60 em RMS for the case with an external FET. Reduction of Surface leakage current One of the major components of the leakage current in drift detectors is due to surface leakage current. Surface leakage is due to electron-hole pair generation at the silicon/oxide interface. Because of the large number of midgap states at the interface, the surface generation component is typically much larger than the bulk component and diffusion current. Surface leakage is strongly dependent on the starting material, the processing conditions, and the operating temperature. The oxidation process is probably the most critical. TCA (trichloroethane) oxidation, which we have used, has been reported to lower generation rates [6]. We measured rates of around 60 na/cm2. One way to control the surface leakage current is to redirect it to a guard anode, preventing it from reaching the main anode [7]. This technique works well with a diffusion spiral [7], because the non-diffused region forms its own spiral which directs charge to a guard anode. In our previous work we introduced a new approach developed to suppress surface generation current. Instead of diverting surface leakage current to a separate guard anode, we have implemented an alternating p+/n+ ring structure to minimize the depleted surface area, which is responsible for the current. If the surface of the detector between p+ diffusion rings is doped n+, it will not be depleted, and only those areas between the n+ and p+ rings will be depleted. MEDIC1 simulations and experimental data show that with this technique the amount of depleted surface area is significantly reduced and the surface leakage generation rates become comparable with the bulk generation rates (1 &cm*) [8]. The additional advantage of the new structure is the robustness of the design to process defects. With the guard anode technique, any defect
954 along the surface current path, resistance variation or large local current generation could render the guard anode ineffective. With the p+/n+ ring design, however, a defect in either p+ or n+ region is likely to be benign. X-Ray Measurements Electronic Noise: Preliminary X-ray measurements were made with a p+/n+ ring SDD structure. The SDD was mounted in a test chamber on a ceramic substrate and slightly cooled with a Peltier cooler to about -10 C to reduce the electronic noise and achieve less than 75 pa leakage current. The test chamber, with a 125 pm thick Beryllium window, was evacuated to prevent water condensation. Electronics with an external FET were used in these measurements. The use of an external FET elevates the noise over what can be achieved optimally with an integrated FET. Figure 2 shows an Fe-55 source spectrum when the whole SDD active area was uniformly illuminated. Energy resolution of 540 ev (FWHM) was obtained at 5.9 kev and electronic noise of 500 ev was measured by the pulser method. Charge Collection Uniformity: An experiment was designed to evaluate the charge collection uniformity over the whole active SDD area. A 0.6 mm diameter collimator was placed directly above the SDD constricting the x-ray beam from an Fe-55 source illuminating the detector to a 2 mm radius. The collimated source was moved above the detector using a precision X-Y stage and the spectral response and the count rate were measured for each point on a square 1 mm step grid. The measured energy resolution was identical (within the statistical error) over the whole active area and equal to that taken with the whole SDD exposed (i.e. without the source collimation). The theoretical and measured (normalized to unity) count rate along the X-axis is shown in Fig. 3. The theoretical count rate characteristic response is the convolution of the detector geometry and the shadow of the collimator on the detector. The observed distribution agrees well with the expected count rate. 7000 6000 :- Fe-55 (entire SDD) 5000 I- g 4000 I- $ 3000 I- 540eVFWHM --, + 2000 :- 1000 :- Pulser 0 2 4 6 8 IO 12 Energy(keV) Figure 2. Fe-55 source x-ray spectrum 1 g 1 5 0.9 d & 0.8 3 0.7 : 0.6 5 8 0.5 21 0.4 d 0.3 2 5 0.2 2 0.1 0 1 3 5 7 9 11 Distance (mm) Figure 3. Count rate measured along X-axis. Theoretical calculation (solid line) and measured response (filled circles).
955 The above results show excellent electron collection from the full detector volume regardless of the drift distance (shortest at the device center and longest at its periphery). Previously reported results have only achieved results for very small drift detector volume (fraction of the full detector volume). Thus, to the best of our knowledge, this is first demonstration of large active volume x-ray SDD at this performance level by anyone. X-Ray Spectra: Figure 4 presents the X-ray fluorescence (XRF) spectra of characteristic K X- ray lines of Titanium (4.5 kev), Iron (6.4 kev), Nickel (7.47 kev), and Copper (8.04 kev). The XRF spectra were taken using a laboratory X-ray excitation source and detection system consisting of X-ray generator tube (with Rhodium anode and Beryllium exit window), target holder and an attachment for the detector test chamber. The first peak in all of the spectra in Fig. 4 is from the XRF target while the second peak is a pulser-induced peak used to measure the electronic noise of the system. The electronic noise level was 500 ev FWHM with the external FET. This agrees well with the theoretical noise (460 ev) calculated using the model described in [4] for total input capacitance of about 3 pf at 1.5 microseconds peaking time. The gaussian spread of the detector signal (electron cloud) in the time domain estimated using measurement of the rise time was found to be less than 35 ns for all drift distances. This number agrees well with the results of theoretical calculations of the drifting electron cloud shape evolution [9]. 0 1 2 3 4 5 6 7 8 9 1011~213141516 0 1 2 3 4 5 6 7 8 9 10111213141516 Energy (kev) Energy (kev) Figure 4: X-ray fluorescence (XRF) spectra of characteristic K X-ray lines of Titanium (4.5 kev), Iron (6.4 kev), Nickel (7.47 kev), and Copper (8.04 kev). The first peak in all spectra is from XRF target while the second peak is the pulser-induced peak.
956 Discussion Use of a particular detector technology for a given practical application depends on many factors. The most important factors include a) energy resolution; b) spectral quality (peak symmetry, tailing, peak-to-background ratio, etc.); c) detection efficiency; d) maximum detector active area, e) maximum count rate, r) detector and input electronics operating temperature; g) stability and reliability; i) ability to reduce weight, size, and power consumption; and j) cost. The requirements for the design and construction of instrumentation for medical applications impose additional restrictions. These additional requirements include a) ease of use (miniaturization), b) maximum solid angle and efficiency to minimize both radiation dose and measurement time, c) low operating bias voltages for safety reasons, and d) capability for sterilization. Currently all the detector technologies that we have discussed (HgI,, CdTe, CdZnTe, Si (p-i-n), and SDD) offer very similar spectral characteristics for the detection of x-rays. Thus the practical selection of a particular detector technology for a given application is dictated by the other requirements. One of these important criterion is the stopping power (inverse of the photoelectric linear attenuation coefficient). Si diodes of thickness 300 pm have a good detection efficiency up to 10 kev (900/,). For higher energies the detection efficiency drops very rapidly making them impractical for the majority of applications above 10 kev. CdTe and HgI, have significantly higher detection efficiency due to their high average atomic numbers. CdTe and HgI, have similar detection efficiencies up to 88 kev with certain differences related specifically to the x-ray absorption of material constituents and their absorption edges. Above 88 kev (Mercury K-absorption edge) the stopping power of HgI, is significantly higher than CdTe due to a higher Z number. Thus, for example, at 140 kev, which corresponds to the gamma-ray energy of the most widely used isotope in nuclear medicine: 99mTc, an easily attainable thickness of < 3 mm of mercuric iodide ensures efficiency of 90% whereas for CdTe almost 7 mm thick detectors are needed in order to obtain the same efficiency. The other differentiating factor in the above detector technologies is the operating temperature. In the case of HgI,, an operating temperature of 0 C is sufficient to obtain excellent spectral results. Cooling below -5 C gives very little benefit since properly fabricated detectors can attain leakage currents of <lo-l3 A at 0 C. Further cooling only reduces input FET noise and eventually enhances charge collection (through an increase in the mobilities of carriers) that can only be seen with thick detectors. SDDs can be operated between -10 C and -20 C. In the case of CdTe, CdZnTe, and Si(p-i-n) the best results are obtained at -40 C. The primary reason for the lower temperature is to keep the dark leakage current below lo-l3 A. The difference in operating temperature has a large impact on the design of hand held portable instrumentation. Whereas truly miniature systems are possible using -5 C detector technologies, systems using -40 C detector technologies will be significantly more bulky, primarily due to additional battery requirements [3]. Future improvements will rely on use of more efficient battery technologies and switched temperature regulators. Another issue of importance for any practical application is the stability and reliability of the detector technology. Silicon technology, of course, has achieved the highest degree of maturity;
957 however, CdTe and HgI, technologies have made great advances with improvements in lattice perfection, a decrease in impurities, and better detector surface passivation and encapsulation. SDDs when completely developed including integrated input FETs offer large active areas >lcm*, unparalleled high count rates >I O6 counts per second due to the theoretical minimum of electronic noise at short peaking times (0.1 ps), and high energy resolution. Currently attainable x-ray spectroscopy performance with the technological approaches discussed above is comparable to that of cryogenically cooled Si[Li] or Ge systems without the need for liquid nitrogen. The parallel development of miniaturized, low-noise, low-power processing electronics allows one to construct very convenient instruments for a variety of applications. The reduced weight, power, and size of the spectrometers makes them extremely attractive as portable x-ray florescence instruments for medical research, environmental monitoring, geological exploration, marine mineral analysis, archeometry, space exploration, and industrial materials quality assurance. ACKNOWLEDGEMENTS This work was supported by the Jet Propulsion Laboratory Contract No. 733525 and by the National Institutes of Health, Grant No. lr43 CA75815 and Grant No. 2R44 RR10647. The authors wish to thank Dr. Julie D. Segal of Stanford University for her contribution to the simulation and design of SDD structures. The valuable technical assistance of Mr. J. Menjivar and Mrs. M. Wells of Photon Imaging, Inc. is greatly appreciated. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. J.S. Iwanczyk, Y.J. Wang, J.G. Bradley, J.M. Conley, A.L. Albee, and T.E. Economou, IEEE Trans. Nucl. Sci. NS-36 (1) (1989) 841-845. A. Niemela and H. Sipila, IEEE Trans. on Nucl. Sci. Vol. 41, No. 4, (1988) 1054. J.S. Iwanczyk, B.E. Patt, Y.J. Wang, and A.Kh. Khusainov, Nucl. Instr. & Meth. in Phys. Res. A 380 (1996) 186-192. J.S. Iwanczyk, B.E. Patt, G. Vilkelis, L. Rehn, J. Metz, B. Hedman & K. Hodgson, Nucl. Instr. & Meth. in Phys. Res. A380 (1996) 288-294. RONTEC GmbH Catalog, June 1997. W. Chen et al., Fabrication of Large Area Si Cylindric Drift Detectors, IEEE Trans. on Nucl. Sci., Vol. 41, No. 4, (1994) 941-947. P. Rehak et al, IEEE Trans. on Nucl. Sci., Vol. 36, No. 1 (1989) 203-209. S. Holland, Nucl Instr. andmeth. in Phys. Res., Vol. A305, (1991) 541-548. J.D. Segal, J.D. Plummer, and C. Kenney, Proceedings 1996 IEEE Nuclear Science Symposium, pp. 558-562.