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Development of diamond radiation detectors for SSC and LHC
NUCLEAR INSTRUMENTS &M IN PHYSICS RESEARCH
Nuclear Instruments and Methods in Physics Research A315 (1992) 39-42 North-Holland
SectionA
Development of diamond radiation detectors for SSC and LHC Presented by S .K. Kim
M. Franklin a, A. Fry b, K.K. Gan c, S. Han d, H. Kagan c, S. Kanda e, D. Kania d, R. Kass c, S.K. Kim f, R. Malchow c, F. Morrow c, S. Olsen e, W.F. Palmer c, L.S. Pan c, F. Sannes =, S. Schnetzer ', R. Stone f , Y. Sugimoto 9, G.B . Thomson f , C. White ' and S. Zhao
Department of Physics, Harvard Universitytyy; Boston, MA 02138, USA Department of Physics, SSC Laboratory, Dallas, TX 75237, USA `Department of Physics, The Ohio State University, Columbus, OH 43210, USA d Laser Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA `' Department of Physics, University of Rochester, Rochester, NY 14627, USA f Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854, USA s KEK, Tsukuba-shi, 1baraki-ken, Japan 305 h
Diamond is a nearly ideal material for use as a radiation detector in the high rate and high radiation environments of the SSC and LHC. The recent development of the chemical vapor deposition (CVD) method of diamond growth promises to make feasible the use of diamond in large quantities. We have carried out beam tests of various samples of CVD diamond supplied by several 1 from .ennm ~m manufacturers and have measured signals signals . .. . +artErleS Details of these meaSWIIl~r__arc Yi.. n_n .b
r
1 . Introduction In order to realize the experimental goals at the planned high energy, high luminosity hadron colliders (SSC and LHC), it is essential to develop detector technologies that are able to handle the extremely high rates and that can operate in the severe radiation environments. Detectors based on diamond could have advantages over all existing technologies because of diamond's inherent radiation hardness, fast charge collection speed, and potential low cost . Table 1 Properties of diamond Property Band gap [eV] Resistivity [S2. cm] Breakdown voltage [V/cm] Electron mobility [cm 2 V -1 S -1 1 Hole mobility [cm 2 V -1 s -1 ] Saturation velocity [g,m/ns] Dielectric constant Neutron transmutation cross section [mb] Energy to create e-h pair [eV] Mass density [g/cm ;] Average min ionizing signal/100 Wm Average min ionizing signal/0.1% X0
Diamond
Silicon
5 .5
107 1800 1200 220 5 .6
1 .1 105 103 1500 500 100 11 .7
3 .2 13 3 .5 3600 e 4500 e
80 3 .6 2 .3 8000 e 7500 e
> loll
IGIIIJ
âSV.
. .,u .7 ca i.u . "
In table 1, we summarize the relevant properties of diamond and compare them with those of silicon .. A detailed description of diamond properties can be found in ref. [1]. Since diamond has a large band gap, 5.5 eV, it is a very good electrical insulator. The resistivity of high-pu -ity diamond is typically 10 11 ft cm to 10' 6 t cm . Because of this large resistivity, a large electric field (> 10 4 V/cm) can be applied across the diamond layer without producing significant leakage current . The charge created by the ionization process is collected, or induced, on ohmic contacts on the surface. Lower resistivity materials, on the other hand, require the use of a reversed biased pn junction to prevent a large leakage current whose fluctuations would dominate the signal . 2 . Charge collection speed As shown in table 1, the breakdown electric field fcr diamond is about 10 7 V/cm. It is, therefore, possible to apply a large enough field to reach the saturation velocity of 2.2 x 10 7 cm/s. In silicon, because of the pn junction and associated depletion region, the maximum field that can be applied before avalanche breakdown is only about 10 3 V/cm . This limits the electron velocity to a few times 10 6 cm/s. Since for either diamond or silicon the required detector thickness is a few hundred microns, the collection time
0168-9002/92/$05 .00 © 1992 - Elsevier Science Publishers B.V . All rights reserved
I . HIGH LUMINOSITY TRACKING
40
AI. Franklin et al. / Diamond radiation detectors for SSC and LHC
would be about 1 ns in diamond compared to about 20 ns and 60 ns for the electrons and holes, respectively, in silicon.
3. Radiation hardness
One of the primary manifestations of radiation damage in solid state detectors is an increase of leakage current due to the production of intraband states in the depletion region caused by the displacement of atoms from lattice sites . These states then act as current generation sources. The leakage current is given by Ileak ^' qAn ; d$, where n; is the intrinsic carrier concentration and 0 is radiation fluence. Due to the absence of a pn junction and the extremely small value of n;, this effect should be negligible in Diamond. Another effect of radiation damage is a decrease in the collected charge due to the production of trapping centers. Because of the relatively large binding energy of atoms in the diamond lattice, the degree of lattice a:~ ., ., in silicon. dislocallon is rluch smalicr in diânionu.i *1, than Also, the neutron transmutation cross section for diamond is more than 20 times smaller than that for silicon . As a result, diamond should be much less sensitive to neutron damage . Although no data for radiation damage on CVD diamond exists, neutron damage on natural diamond was reported by Kozlov et al . [2]. They showed that a diamond detector can survive up to a few times 10 14 neurons/cm -. In another study, no increase in leakage current was observed for exposures up to 10 r6"/ cm- of 1.5 MeV electrons in a Schottky diode made of diamond [3].
4. CVD diamonds Natural diamond has been used as a radiation detcctor since 1949 [4], However, it is extremely expensive and cannot be used on a large scale. Also, it contains naturally high level of impurities, such as nitrogen, and crystal defects which limit its carrier lifetime . The recently developed technology of manufacturing diamond by chemical vapor deposition (CVD) [5] has the potential of aaowing the economical production of diamond in large sheets and of significantly higher purity than natural diamond. In the CVD process, a hydrocarbon gas, such as methane, is mixed with a large concentration o: molecular hydrogen gas. The gas mixture is then excited by either microwaves, a hot filament, or some other en-
ergy source . The resulting reactive gas mixture is brought into contact with a substrate, typically silicon, where the carbon based radicals are reduced and linked together with single bonds (sp; hybridized orbitals) forming a diamond crystal. Because of the large number of variables, systematic correlations between growth parameters and diamond quality is one of the main tasks in developing CVD diamond detectors. 5. Beam tests of diamond We have carried out beam tests of single-crystal natural and polycrystalline CVD diamonds at the accumulator ring (AR) test beam at KEK in Japan and at the M13 beam line at TRIUMF in Canada . The AR is an 8 GeV storage ring which is a part of the injection system of TRISTAN. By use of a thin internal target, scattered electrons with energies from 0.5 GeV to 5.5 GeV at rates of a few Hz can be obtained . At TRIUMF the M13 beam line is a tuneable 100 MeV secondary beam line consisting of pions, muons and electrons. At this momentum, minimum ionizing (e's), twice minimum ionizing (IQ's), and three times minimum ionizing (,rr's) particles are available at rates up to 10 6 particles per second . The samples tested at KEK and TRIUMF included natural diamond (D34) and polycrystalline CVD diamond . The natural diâmond sample was 1 mm thick (1 x 3 mm'- area) and contained the natural abundance of nitrogen (10-30 ppm) . The CVD diamond samples were grown by five manufacturers using a variety of growth processes. Under scanning electron microscopy, most samples showed a typical needle-like growth of diamond crystals extending across the layer. Some samples were polished to remove surface roughness from the growth process. Sample thicknesses ranged between 100-500 [Lm. The total number of samples tested was about 50 . Scintillation counters plus a small (3 x 3 mm'`) silicon detector defined the incident beam and formed the trigger. A removable lead stack was used to create electromagnetic showers (used at KEK) and thereby increase the numbers of particles traversing the detectors. Signals from a diamond detector and from a silicon detector located just downstream of the diamond were measured and compared . These signals were amplified by a charge sensitive preamplifier [6] and by a shaping amplifier. Fig. 1 shows minimum ionizing signals as seen by the natural diamond detector and by the silicon detector . Fig. 2 shows the corresponding signal from a CVD diamond detector and silicon detector . The rise time and shapes of these pulses are determined by the shaping amplifier. The diamond signals themselves have rise times of less than I ns.
M. Franklin et al. / Diamond radiation detectors for SSC and LHC
41
60
Diamond
Silicon
i
N N
Time (5 ixsec/div)
Fig . 1 . Minimum ionizing single pulses in natural diamond (upper trace) and silicon (lower trace) . The diamond to silicon pulse height ratio on this particular event was 1 :2 . The vertical scale is 20 mV/division.
The quantity used to characterize the diamond in these tests is the charge collection distance . The observed signal results from induced charge on the electrodes due to the movement of the electrons and holes . Because of impurities and defects, the electrons and holes drift on average some distance d before capture . When this distance is much smaller than the diamond thickness, the observed signal is proportional to the ratio of d to the diamond thickness t . All samples tested to date have fallen into this category . The collection distance can then be determined by: Qmeas/Qgen = d/t .
Diamond .
0T cri
t
a
Silicon
MIA
0
t 1
Time (5 ,sec/div)
Fig . 2. Minimum ionizing single pulses in CND diamond (upper trace) and silicon (lower trace). The diamond to silicon pulse height ratio on this particular event was 1 :8 . For the diamond trace, the vertical scale its 10 mV/division . For the silicon trace , it iç 5O niV /division .
10
20
30
40
50
Electric Field (kWcm)
Fig . 3. Collection distances in natural and CVD diamonds.
Here Qgen is the amount of charge generated by *he ionizing radiation and Q me;,, is the measured charge . The generated charge in diamond has been calculated in two ways: 1) by normalizing the diamond pulse: height to the silicon pulse height after correction for solid angle and dE/dx, and 2) using the EGS Monte Carlo. Both methods agree . In these cclculations, we used 13 eV of energy deposit per electron-hole pair produced . The collection distance is also related to the mobility tL, carrier lifetime T, and applied electric field E by:
d =I - T = tL ET . Fig . 3 compares the collection distance for natural diamond and four CVD samples . At electric fields of 25 kV/cm, the natural diamond reaches a collection distance of 50-60 Wm and is still not saturated . At this electric field the signal to noise (SIN) is approximately 6 :1 . Extrapolating from this data we expect the natural diamond to reach collection distances of approximately 100 Wm (SIN = 10 : 1) at electric fields of 40 kV/cm . At low electric field, one of the CVD diamonds compares well with natui ai diamond . This is the region where impurities are expected to dominate . As shown in fig . 3, at higher fields this CVD sample saturates at a collection distance around 10-15 pm ; significantly lower than that of the natural diamond . The onset of this saturation is probably related to imerfections and defects in the CVD sample . This 1 . HIGH LUMINOSITY TRACKING
M. Franklin et al. / Diamond radiation detectors for SSC and LHC
42
T-1-M .
1 .4
6 . Conclusion CVD Diamond
The experiments des ribed above were the first demonstration that CVD diamond can detect charged particle minimum ionizi g radiation and handle large rates . The collection dist nce is of the order of 10 wm in CVD diamond and 0 p .m in natural diamond . These preliminary resul s based on "off the shelf CVD diamond are very encouraging . We are at the point now where calorim try is becoming feasible and tracking is within reach .
E = 20 kV/cm
s
1 .2
m
a m
1 .0
e
e
Acknowledgements
co
m
0 .8
0 .6
10 10 , 3 4 10 10 2
` JS
10 6
Rate (HiICM 2) Fig. 4. Pulse height as a function of rate in CVD diamond. The pulse heights are normalized to 1 .0 for the lowest rate point. effect is being studied and correlated with CVD production parameters . In addition to studying the charge collection in natural and CVD diamond, we have begun to look at rate effects. In fig . 4, we show the normalized pulse height as a function of rate in CVD diamond . No degradation of pulse height was observed up to rates of 10 4 particles cm -2 s -1 . This is the first indication that diamond will withstand the large rate environment of the SSC.
We are grateful for he cooperation we received from both the KEK and TRIUMF laboratories which made these test beam stu ies possible . We also wish to thank Crystallume Inc., eneral Electric, Norton Co ., Sumitomo, and Westinghouse for providing the diamonds used in these tests . This work was supported in part by grant #RGFY9164 from the Texas National Research Laboratory Commission (#RGFY9164) and from the SSC. In addition, two of us, K.K . Gan and S .K. Kim, would like to acknowledge support of the SSC Fellowship Program funded by the Texas National Research Laboratory Commission . References [1) R. Piano et al ., SSC-EOI0009. [2] S .F. Kozlov et al ., IEEE Trans . Nucl . Sci . NS-22 (1975) 160 . [3] M . Geis, MIT Lincoln Laboratory, private communication . [4] R . Hofstadter, Nucleonics 4 (1949), 2 (April) and 29 (May). [51 J . Angus and C . Hayman, Science 241 (1988) 913 ; and W . Yarbrough and R . Messier, Science 247 (1990) 913 . [6] T . Taniguchi, Y . Fukushima and Y . Yoribayashi, IEEE Trans. Nucl . Sci . NS-36 (1989) 657.
Nuclear Instruments and Methods in Physics Research A315 (1992) 39-42 North-Holland
SectionA
Development of diamond radiation detectors for SSC and LHC Presented by S .K. Kim
M. Franklin a, A. Fry b, K.K. Gan c, S. Han d, H. Kagan c, S. Kanda e, D. Kania d, R. Kass c, S.K. Kim f, R. Malchow c, F. Morrow c, S. Olsen e, W.F. Palmer c, L.S. Pan c, F. Sannes =, S. Schnetzer ', R. Stone f , Y. Sugimoto 9, G.B . Thomson f , C. White ' and S. Zhao
Department of Physics, Harvard Universitytyy; Boston, MA 02138, USA Department of Physics, SSC Laboratory, Dallas, TX 75237, USA `Department of Physics, The Ohio State University, Columbus, OH 43210, USA d Laser Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA `' Department of Physics, University of Rochester, Rochester, NY 14627, USA f Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854, USA s KEK, Tsukuba-shi, 1baraki-ken, Japan 305 h
Diamond is a nearly ideal material for use as a radiation detector in the high rate and high radiation environments of the SSC and LHC. The recent development of the chemical vapor deposition (CVD) method of diamond growth promises to make feasible the use of diamond in large quantities. We have carried out beam tests of various samples of CVD diamond supplied by several 1 from .ennm ~m manufacturers and have measured signals signals . .. . +artErleS Details of these meaSWIIl~r__arc Yi.. n_n .b
r
1 . Introduction In order to realize the experimental goals at the planned high energy, high luminosity hadron colliders (SSC and LHC), it is essential to develop detector technologies that are able to handle the extremely high rates and that can operate in the severe radiation environments. Detectors based on diamond could have advantages over all existing technologies because of diamond's inherent radiation hardness, fast charge collection speed, and potential low cost . Table 1 Properties of diamond Property Band gap [eV] Resistivity [S2. cm] Breakdown voltage [V/cm] Electron mobility [cm 2 V -1 S -1 1 Hole mobility [cm 2 V -1 s -1 ] Saturation velocity [g,m/ns] Dielectric constant Neutron transmutation cross section [mb] Energy to create e-h pair [eV] Mass density [g/cm ;] Average min ionizing signal/100 Wm Average min ionizing signal/0.1% X0
Diamond
Silicon
5 .5
107 1800 1200 220 5 .6
1 .1 105 103 1500 500 100 11 .7
3 .2 13 3 .5 3600 e 4500 e
80 3 .6 2 .3 8000 e 7500 e
> loll
IGIIIJ
âSV.
. .,u .7 ca i.u . "
In table 1, we summarize the relevant properties of diamond and compare them with those of silicon .. A detailed description of diamond properties can be found in ref. [1]. Since diamond has a large band gap, 5.5 eV, it is a very good electrical insulator. The resistivity of high-pu -ity diamond is typically 10 11 ft cm to 10' 6 t cm . Because of this large resistivity, a large electric field (> 10 4 V/cm) can be applied across the diamond layer without producing significant leakage current . The charge created by the ionization process is collected, or induced, on ohmic contacts on the surface. Lower resistivity materials, on the other hand, require the use of a reversed biased pn junction to prevent a large leakage current whose fluctuations would dominate the signal . 2 . Charge collection speed As shown in table 1, the breakdown electric field fcr diamond is about 10 7 V/cm. It is, therefore, possible to apply a large enough field to reach the saturation velocity of 2.2 x 10 7 cm/s. In silicon, because of the pn junction and associated depletion region, the maximum field that can be applied before avalanche breakdown is only about 10 3 V/cm . This limits the electron velocity to a few times 10 6 cm/s. Since for either diamond or silicon the required detector thickness is a few hundred microns, the collection time
0168-9002/92/$05 .00 © 1992 - Elsevier Science Publishers B.V . All rights reserved
I . HIGH LUMINOSITY TRACKING
40
AI. Franklin et al. / Diamond radiation detectors for SSC and LHC
would be about 1 ns in diamond compared to about 20 ns and 60 ns for the electrons and holes, respectively, in silicon.
3. Radiation hardness
One of the primary manifestations of radiation damage in solid state detectors is an increase of leakage current due to the production of intraband states in the depletion region caused by the displacement of atoms from lattice sites . These states then act as current generation sources. The leakage current is given by Ileak ^' qAn ; d$, where n; is the intrinsic carrier concentration and 0 is radiation fluence. Due to the absence of a pn junction and the extremely small value of n;, this effect should be negligible in Diamond. Another effect of radiation damage is a decrease in the collected charge due to the production of trapping centers. Because of the relatively large binding energy of atoms in the diamond lattice, the degree of lattice a:~ ., ., in silicon. dislocallon is rluch smalicr in diânionu.i *1, than Also, the neutron transmutation cross section for diamond is more than 20 times smaller than that for silicon . As a result, diamond should be much less sensitive to neutron damage . Although no data for radiation damage on CVD diamond exists, neutron damage on natural diamond was reported by Kozlov et al . [2]. They showed that a diamond detector can survive up to a few times 10 14 neurons/cm -. In another study, no increase in leakage current was observed for exposures up to 10 r6"/ cm- of 1.5 MeV electrons in a Schottky diode made of diamond [3].
4. CVD diamonds Natural diamond has been used as a radiation detcctor since 1949 [4], However, it is extremely expensive and cannot be used on a large scale. Also, it contains naturally high level of impurities, such as nitrogen, and crystal defects which limit its carrier lifetime . The recently developed technology of manufacturing diamond by chemical vapor deposition (CVD) [5] has the potential of aaowing the economical production of diamond in large sheets and of significantly higher purity than natural diamond. In the CVD process, a hydrocarbon gas, such as methane, is mixed with a large concentration o: molecular hydrogen gas. The gas mixture is then excited by either microwaves, a hot filament, or some other en-
ergy source . The resulting reactive gas mixture is brought into contact with a substrate, typically silicon, where the carbon based radicals are reduced and linked together with single bonds (sp; hybridized orbitals) forming a diamond crystal. Because of the large number of variables, systematic correlations between growth parameters and diamond quality is one of the main tasks in developing CVD diamond detectors. 5. Beam tests of diamond We have carried out beam tests of single-crystal natural and polycrystalline CVD diamonds at the accumulator ring (AR) test beam at KEK in Japan and at the M13 beam line at TRIUMF in Canada . The AR is an 8 GeV storage ring which is a part of the injection system of TRISTAN. By use of a thin internal target, scattered electrons with energies from 0.5 GeV to 5.5 GeV at rates of a few Hz can be obtained . At TRIUMF the M13 beam line is a tuneable 100 MeV secondary beam line consisting of pions, muons and electrons. At this momentum, minimum ionizing (e's), twice minimum ionizing (IQ's), and three times minimum ionizing (,rr's) particles are available at rates up to 10 6 particles per second . The samples tested at KEK and TRIUMF included natural diamond (D34) and polycrystalline CVD diamond . The natural diâmond sample was 1 mm thick (1 x 3 mm'- area) and contained the natural abundance of nitrogen (10-30 ppm) . The CVD diamond samples were grown by five manufacturers using a variety of growth processes. Under scanning electron microscopy, most samples showed a typical needle-like growth of diamond crystals extending across the layer. Some samples were polished to remove surface roughness from the growth process. Sample thicknesses ranged between 100-500 [Lm. The total number of samples tested was about 50 . Scintillation counters plus a small (3 x 3 mm'`) silicon detector defined the incident beam and formed the trigger. A removable lead stack was used to create electromagnetic showers (used at KEK) and thereby increase the numbers of particles traversing the detectors. Signals from a diamond detector and from a silicon detector located just downstream of the diamond were measured and compared . These signals were amplified by a charge sensitive preamplifier [6] and by a shaping amplifier. Fig. 1 shows minimum ionizing signals as seen by the natural diamond detector and by the silicon detector . Fig. 2 shows the corresponding signal from a CVD diamond detector and silicon detector . The rise time and shapes of these pulses are determined by the shaping amplifier. The diamond signals themselves have rise times of less than I ns.
M. Franklin et al. / Diamond radiation detectors for SSC and LHC
41
60
Diamond
Silicon
i
N N
Time (5 ixsec/div)
Fig . 1 . Minimum ionizing single pulses in natural diamond (upper trace) and silicon (lower trace) . The diamond to silicon pulse height ratio on this particular event was 1 :2 . The vertical scale is 20 mV/division.
The quantity used to characterize the diamond in these tests is the charge collection distance . The observed signal results from induced charge on the electrodes due to the movement of the electrons and holes . Because of impurities and defects, the electrons and holes drift on average some distance d before capture . When this distance is much smaller than the diamond thickness, the observed signal is proportional to the ratio of d to the diamond thickness t . All samples tested to date have fallen into this category . The collection distance can then be determined by: Qmeas/Qgen = d/t .
Diamond .
0T cri
t
a
Silicon
MIA
0
t 1
Time (5 ,sec/div)
Fig . 2. Minimum ionizing single pulses in CND diamond (upper trace) and silicon (lower trace). The diamond to silicon pulse height ratio on this particular event was 1 :8 . For the diamond trace, the vertical scale its 10 mV/division . For the silicon trace , it iç 5O niV /division .
10
20
30
40
50
Electric Field (kWcm)
Fig . 3. Collection distances in natural and CVD diamonds.
Here Qgen is the amount of charge generated by *he ionizing radiation and Q me;,, is the measured charge . The generated charge in diamond has been calculated in two ways: 1) by normalizing the diamond pulse: height to the silicon pulse height after correction for solid angle and dE/dx, and 2) using the EGS Monte Carlo. Both methods agree . In these cclculations, we used 13 eV of energy deposit per electron-hole pair produced . The collection distance is also related to the mobility tL, carrier lifetime T, and applied electric field E by:
d =I - T = tL ET . Fig . 3 compares the collection distance for natural diamond and four CVD samples . At electric fields of 25 kV/cm, the natural diamond reaches a collection distance of 50-60 Wm and is still not saturated . At this electric field the signal to noise (SIN) is approximately 6 :1 . Extrapolating from this data we expect the natural diamond to reach collection distances of approximately 100 Wm (SIN = 10 : 1) at electric fields of 40 kV/cm . At low electric field, one of the CVD diamonds compares well with natui ai diamond . This is the region where impurities are expected to dominate . As shown in fig . 3, at higher fields this CVD sample saturates at a collection distance around 10-15 pm ; significantly lower than that of the natural diamond . The onset of this saturation is probably related to imerfections and defects in the CVD sample . This 1 . HIGH LUMINOSITY TRACKING
M. Franklin et al. / Diamond radiation detectors for SSC and LHC
42
T-1-M .
1 .4
6 . Conclusion CVD Diamond
The experiments des ribed above were the first demonstration that CVD diamond can detect charged particle minimum ionizi g radiation and handle large rates . The collection dist nce is of the order of 10 wm in CVD diamond and 0 p .m in natural diamond . These preliminary resul s based on "off the shelf CVD diamond are very encouraging . We are at the point now where calorim try is becoming feasible and tracking is within reach .
E = 20 kV/cm
s
1 .2
m
a m
1 .0
e
e
Acknowledgements
co
m
0 .8
0 .6
10 10 , 3 4 10 10 2
` JS
10 6
Rate (HiICM 2) Fig. 4. Pulse height as a function of rate in CVD diamond. The pulse heights are normalized to 1 .0 for the lowest rate point. effect is being studied and correlated with CVD production parameters . In addition to studying the charge collection in natural and CVD diamond, we have begun to look at rate effects. In fig . 4, we show the normalized pulse height as a function of rate in CVD diamond . No degradation of pulse height was observed up to rates of 10 4 particles cm -2 s -1 . This is the first indication that diamond will withstand the large rate environment of the SSC.
We are grateful for he cooperation we received from both the KEK and TRIUMF laboratories which made these test beam stu ies possible . We also wish to thank Crystallume Inc., eneral Electric, Norton Co ., Sumitomo, and Westinghouse for providing the diamonds used in these tests . This work was supported in part by grant #RGFY9164 from the Texas National Research Laboratory Commission (#RGFY9164) and from the SSC. In addition, two of us, K.K . Gan and S .K. Kim, would like to acknowledge support of the SSC Fellowship Program funded by the Texas National Research Laboratory Commission . References [1) R. Piano et al ., SSC-EOI0009. [2] S .F. Kozlov et al ., IEEE Trans . Nucl . Sci . NS-22 (1975) 160 . [3] M . Geis, MIT Lincoln Laboratory, private communication . [4] R . Hofstadter, Nucleonics 4 (1949), 2 (April) and 29 (May). [51 J . Angus and C . Hayman, Science 241 (1988) 913 ; and W . Yarbrough and R . Messier, Science 247 (1990) 913 . [6] T . Taniguchi, Y . Fukushima and Y . Yoribayashi, IEEE Trans. Nucl . Sci . NS-36 (1989) 657.