BULK GaAs-BASED NEUTRON DETECTORS ... - Semantic Scholar

Proceedings of ICONE 8 th 8 International Conference on Nuclear Engineering April 2-6, 2000, Baltimore, MD USA

ICONE – 8827 BULK GaAs-BASED NEUTRON DETECTORS FOR SPENT FUEL ANALYSIS Douglas S. McGregor 2355 Bonisteel Blvd. Nuclear Engin. & Rad. Sci. University of Michigan Ann Arbor, MI 48109 Ph: 734-647-8964 Fax: 734-763-4540 [email protected]

John T. Lindsay 2301 Bonisteel Blvd. Phoenix Memorial Lab University of Michigan Ann Arbor, MI 48109 Ph: 734-936-1583 [email protected]

KEYWORDS Gallium Arsenide, Neutron Detection, Semiconductor ABSTRACT Bulk GaAs-based 10B coated detectors are under investigation as potential devices to assist in transmission analysis of spent nuclear fuel elements. The GaAs detectors are presently being investigated for their radiation hardness, neutron sensitivity and gamma ray insensitivity. The devices are to be used for evaluation, at first, with thermal neutrons, and later with 24 keV neutrons. Preliminary results and design issues are presented. INTRODUCTION Some non-destructive techniques presently used to evaluate spent fuel assemblies include activation analysis (Lokosi, 1990; Nemeth, 1990), passive and interrogative neutron counting (Wurz, 1991), and passive neutron coincidence counting (Hong, 1996). None of these schemes mentioned offer a straightforward method of performing spatial mapping and tomographic imaging of the spent fuel. Information gained from tomographic maps of spent fuel can be used for fuel burnup documentation, safeguards issues, verification of theoretical reactor models, and documentation of fuel cycle and fuel behavior. Generally, tomographic mapping of fuel elements has been impractical for several reasons, the most obvious being time considerations and detector response in the harsh radiation environment. A device capable of detecting neutron events while being naturally insensitive to gamma rays can be used to help reduce dead time effects from the high radiation environment. Gas detector systems (such as 3He filled

Yong-Hong Yang 2355 Bonisteel Blvd. Nuclear Engin. & Rad. Sci. University of Michigan Ann Arbor, MI 48109 [email protected]

John C. Lee 2355 Bonisteel Blvd. Nuclear Engin. & Rad. Sci. University of Michigan Ann Arbor, MI 48109 Ph: 734-764-9379 Fax: 734-763-4540 [email protected]

detectors) seem to operate well for such measurements (Wurtz, 1991). However, gas detectors do not offer the advantage of high spatial resolution. Foil activation at specific fuel bundle locations can be used to determine the distribution of fuel burnup, however the time required for such an operation can extend over a period of several days for a single fuel bundle. A portable detector and detection system are needed that can rapidly provide spatial information on fuel element burnup, withstand the harsh radiation environment, and discriminate between neutron interactions and gamma-ray interactions. A portable system based on solid state GaAs neutron detecting arrays coupled with an iron filtered Sb/Be 24 keV neutron source is presently under development as a tool to evaluate and provide tomographic images of spent fuel elements. Presented in the following manuscript is a description of the GaAs-based neutron detectors and their present status. THEORETICAL CONSIDERATIONS 1. THE TRUNCATED ELECTRICAL FIELD EFFECT GaAs is a compound semiconductor with a room temperature band gap energy of 1.43 eV (McGregor, 1995). Its average ionization energy is 4.3 eV/ electron-hole pair and its average density is 5.32 g/cm3 (McGregor, 1995). Theoretically, the wide band gap ensures that radiation detectors fabricated from bulk GaAs material can be operated at room temperature. However, unintentional impurities in the bulk material render GaAs more conductive than expected under ideal conditions, hence compensation centers are purposely added to increase the material resistivity. Often the residual background dopant impurities render GaAs material p-type, thereby requiring the addition of deep donor compensation centers to increase the resistivity. Presently, the most often used deep donor level

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Copyright © 2000 by ASME

compensation center is the native defect labeled EL2, an antisite defect produced when the arsenic is added at slightly greater concentrations than gallium in the crystal growth process (Holmes, 1982). The EL2 site appears deep in the band gap at approximately 0.8 eV below the conduction band edge. Reverse biased radiation diode detectors fabricated from bulk semi-insulating (SI) GaAs that has been compensated with the EL2 native defect undergoes a very odd effect when operated under a voltage bias. Unlike the traditional V dependence that detector active region widths display when analyzed in most semiconductor physics texts (Sze, 1981), the active region of reverse biased SI GaAs Schottky style diodes demonstrates an almost linear increase with applied voltage, requiring approximately 1 volt/µm of active region width (McGregor, 1992; Berwick, 1993; McGregor, 1994). The odd effect was recently discovered (McGregor, 1992) and analyzed both experimentally and theoretically (McGregor, 1992; Berwick, 1993; McGregor, 1994). The effect causes the appearance of two very distinct operating regions in the diode; (1) a region of fairly constant high electric field and (2) a region of fairly constant low (almost zero) electric field. Presently, it is believed that the “truncated electric field effect” is due to a dynamic change in electron capture cross section of the electrically active deep donor EL2 centers that are located deep within the energy gap of GaAs material (McGregor, 1994). The capture cross section undergoes a dramatic increase at high electric fields, but relaxes back to its previous state under low electric fields. Hence, a transition region exists between the high and low electric field regions in which the electron capture cross section is undergoing changes, bringing about an unstable region, which has been observed to produce low frequency oscillations (Holanyak, 1963; Kaminska, 1982; Derhacobian, 1991). Fortunately, the low frequency oscillations do not appear to interfere with radiation measurements, and GaAs radiation detectors have been used for charged-particle (McGregor, 1994) gamma-ray (McGregor, 1996a), and neutron detection (McGregor, 1996b; McGregor, 2000a; Klann, 2000). Unfortunately, the truncated electric field effect creates a situation in which bulk GaAs is rather poor for gamma-ray detectors since a simple 1 mm thick device would require 1 kV or more to become fully active. However, since the electric field magnitude is approximately 104 V/cm in the high electric field region, and the high electric field region width can be produced with very low voltages (10-50 volts), bulk GaAs devices work very well for charged-particle detectors. The operating voltage can be kept low such that the active region is just wide enough to absorb all of the energy of the charged-particles under investigation while being too thin to absorb a significant number of background gamma-rays. Hence, bulk GaAs detectors can be designed to detect charged-particles while selfdiscriminating out gamma-rays. The low field region width can be kept much greater than the high field region width while maintaining low capacitance. Hence, a device can be 300 µm

thick, retain high resistivity and low capacitance, but have only a 15 µm wide electrically active region. The present work takes advantage of the effect. Schottky contact pixel arrays use radiation hard diode construction (McGregor, 1996b) to produce diode detectors out of EL2 compensated SI bulk GaAs material. The diode arrays are coated with a dielectric material and then coated with 98% pure 10 B material. The 10B upon absorbing neutrons emits chargedparticle reaction products that may enter the detector pixel array. By maintaining a low voltage (10-50 volts), the detector pixels will detect the charged-particle reaction products from neutron events, but will not detect most of the background gamma rays. Hence, the devices will detect neutrons and naturally discriminate away background gamma-ray interference. 2. NEUTRON DETECTION EFFICIENCY – THERMAL (0.0259 eV) AND 24 keV ENERGIES Semiconductor detectors coated with neutron reactive materials offer an alternative approach to scintillator-based neutron imaging devices for neutron radiography. There are various candidate semiconductor materials, including Si, GaAs, and diamond, all of which have advantages and disadvantages (Rose, 1967; Feigl, 1968; Mireshghi, 1994; McGregor, 1996b; Foulon, 1998). Si and bulk GaAs-based devices operate at moderately low voltages, whereas diamond-based films require hundreds of volts to operate. Although diamond-based films appear to be more radiation hard than GaAs, GaAs devices have demonstrated excellent radiation hardness for the present application (McGregor, 1996b). Some of the most common isotopes used for neutron detection converters are 157Gd, 6Li, and 10B. It is true that 157Gd has a much higher thermal neutron absorption cross section than 10 B and 6Li, but the 157Gd(n,γ)158Gd reaction produces low energy gamma-rays and conversion electron reaction products, hence neutron induced events would be difficult to discriminate from the background gamma-rays present in the harsh radiation environment that accompanies fuel elements. Background gamma rays are less likely to interact in a diamond or Si detector than in GaAs, but previous results have shown that the gamma-ray background interference for 10B-coated GaAs detectors is low enough to discriminate between neutron and gamma-ray events. As a result, 10B-coated GaAs detectors offer a good compromise for the desired detector properties. The “effective range” L is the maximum distance that a particle may travel in the neutron reactive film before its energy reduces below the detectable limit preset by the electronic lower level discriminator (LLD). The reaction products lose energy as they move through the neutron reactive film. Reaction product energy self-absorption reduces the energy transferred to the semiconductor detector, thereby affecting the maximized film thickness deposited on the detector. The overall construction of a single pixel for the detector system is illustrated in Fig. 1.

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Sc hottky Contact

Neutron

Lithium-7 Ion

----- --- -- --

Boron-10 Film

Alpha Particle

Back Contact Layer

+ + + + + ++ + + + ++ + ++

Alumina Film

SI GaAs Active Region

SI Bulk GaAs Substrate

Voltage Applied

Connec ted to Pream plifier

Figure 1: The basic construction of a 10B-coated SI bulk GaAs neutron detector. Neutrons interact in the 10B film, thereby releasing an alpha particle and a 7Li ion in opposite directions. Only one particle from the interaction can enter the detector. The excited free charge carriers are swept from the active region by an externally applied bias voltage, and the induced charge is measured by an external preamplifer circuit.

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Transmitted Energy (keV)

The microscopic absorption cross section for 6Li is less than that of 10B, and 6Li has the added disadvantage of being chemically reactive with its surroundings unless precautions are taken. However, 6LiF is a stable compound and is often used for neutron detection purposes. The charged-particle energies from 6 Li(n,α)3H reactions are greater than those emitted from 10 B(n,α)7Li reactions, yet the ΣL values (McGregor, 2000b) differ only slightly between reaction products from 6LiF and reaction products from 10B. Hence the optimized film thickness for 6LiF is over ten times greater than needed for 10B while producing only a slight increase in neutron detection efficiency. In the present application, 10B was chosen as the converter film over 6Li. The thermal neutrons (0.0259 eV) absorbed by 10B produce energetic particles that are emitted at a 180o angle. The primary 10 B(n,α)7Li reaction (94%) results in the emission of a 1.47 MeV alpha particle and a 840 keV 7Li ion in its first excited state, which rapidly de-excites to the ground state (~10-13 seconds) by releasing a 480 keV gamma ray. For the remaining 10 B(n,α)7Li reactions, the 7Li ion reduces directly to its ground state resulting in the emission of a 1.777 MeV alpha particle and a 1.015 MeV 7Li ion (Knoll, 2000). The average range for a 840 keV 7Li ion in boron is 1.6 µm, and the average range for a 1.47 MeV alpha particle is 3.6 µm (Ziegler, 1998). The thermal neutron microscopic absorption cross section for pure 10B is 3840 barns (Knoll, 2000). The microscopic thermal neutron absorption cross section dependence is proportional to the inverse of the neutron velocity (1/v) over much of the energy range (Garber, 1976; McLane, 1988). Pure 10B has an

1.470 MeV α Particle

1800 1600 1400

1.777 MeV α Particle

1200 1000 800 600

1.015 MeV 7Li Ion

400 840 keV 7Li Ion

200 0 0

1

2

3

Interaction Location from Detector, (in µm)

4 10

B Film

Figure 2: Remaining energy of transmitted reaction products from the 10B(n,α)7Li reaction as a function of interaction depth in the 10B material from the contact interface. Contact attenuation was not considered. The calculation was performed for an entrance angle of 90o. atomic density of 1.3 x 1023 atoms/cm3, yielding a macroscopic absorption cross section (Σ) of 500 cm-1. The energy absorbed in the detector is simply the original particle energy minus the combined energy lost in the boron film and the detector contact during transit. Assuming that energy loss in the detector contact is negligible, Fig. 2 shows the energy retained by either charged-particle as a function of transit length through the boron film. At any reaction location within the 10B film, maximum detector entrance energy will be retained by either particle should they enter the detector in an orthogonal trajectory. For instance, if the interaction occurs in the 10B film at a distance of 0.5 µm away from the detector, the maximum energy retained by the 7Li ion will be 430 keV, and the maximum energy retained by the alpha particle will be 1150 keV. For the same interaction distance of 0.5 µm from the detector, the energy retained by the particle when it reaches the detector decreases as the angle increases from orthogonal (0o). Given a predetermined minimum detection threshold (or LLD setting), the effective range L for either particle can be determined. For instance, as shown in Fig. 2, an LLD setting of 300 keV yields LLi = 0.810 µm and Lα = 2.648 µm. A commonly used geometry involves the use of a planar semiconductor detector over which a neutron reactive film has been deposited. Assuming that the neutron beam is perpendicular to the detector front contact, the sensitivity contribution for a reaction product species can be calculated by integrating the product of the interaction probability and the fractional solid angle over the absorber 10B film thickness DF (McGregor, 1996b; 2000a; 2000b):

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ìïæ D üï 1 ö ÷÷ 1 − e- Σ F DF − F ý ; DF ≤ L, (1A) S p ( DF ) = 0.5 Fp íçç1 + L ïþ ïîè Σ F L ø

(

)

0.010

S p ( DF ) = 0. 5 F p e

- Σ F ( D F - L)

24 keV Neutron Detection Efficiency (%)

and

üï ìïæ 1 ö ÷÷ 1 − e - Σ F L − 1ý ; íçç1 + ïþ ïîè Σ F L ø DF > L, (1B)

(

)

where Fp refers to the branching ratio of the reaction product emissions. The total sensitivity accordingly can be found by adding all of the reaction product sensitivities (McGregor, 2000a; 2000b):

0.009 0.008 0.007 0.006 0.005 0.004 0.003 0.002 0.001 0.000 0

N

S ( DF ) Total = å S p ( DF ) ,

where N is the number of different reaction product emissions. In the case of 10B-based films, N equals 4. Notice from equation 1B that the value of Sp reduces as DF becomes larger than the value of L. As a result of this, there will be an optimum neutron reactive film thickness for front-irradiated detectors (see Fig. 3). Since the minimum particle detection threshold determines L, the optimum film thickness is also a function of the LLD setting. From Fig. 3, with the LLD set at 300 keV, the maximum achievable thermal neutron detection efficiency is 4%. The thermal neutron detection efficiency can be increased to 4.8% by lowering the LLD setting (McGregor, 1996b), but only at the expense of accepting more system noise and gamma-ray background interference.

Thermal Neutron Detection Efficiency (%)

4.5

Total for the

2

3

4

5

6

10

B Film Thickness (µm)

(2)

p =1

1

Figure 4: Calculated detection efficiency for a 10B-coated semiconductor detector. Energy attenuation in the contact region was not included. Shown is the expected detection efficiency for 24 keV neutrons. LLD = 300 keV for the calculations. It is planned that a Sb/Be neutron source be used to accompany the final version of the portable unit. Sb/Be neutron sources emit 24 keV neutrons, for which 10B has only a 6 barn microscopic absorption cross section, thereby reducing the detection efficiency of the device below that expected for thermal neutrons. The macroscopic cross section for 10B at 24 keV is 0.78 cm-1. Figure 4 shows the calculated 24 keV neutron detection efficiency for a 10B-coated detector as a function of film thickness, indicating that the maximum expected efficiency is approximately 0.007%.

4.0 10B(n,α)7Li

reaction

3.5

1.47 MeV α particle

3.0 2.5

EXPERIMENTAL RESULTS 1. PRELIMINARY DEVICE DESIGN

840 keV 7Li ion

2.0

1.777 MeV 1.015 MeV α particle 7Li ion

1.5 1.0 0.5 0.0 0

1

2 10

3

4

5

6

B Film Thickness (µm)

Figure 3: Calculated thermal neutron detection efficiency for a 10 B-coated semiconductor detector. Energy attenuation in the contact region was not included. Shown are the contributions from the four charged-particle emissions and the total cumulative thermal neutron detection efficiency. LLD = 300 keV for the calculations.

The first generation devices have been designed such that eventual reduction to commercial fabrication will be straightforward. The devices consist of commercial grade bulk SI GaAs wafers that have been thinned to only 200 µm overall width. The wafer front and backsides were lapped with alumina slurry and then polished with a bromine/methanol solution to achieve a highly polished and specular finish. Each device has a n-type ohmic contact, composed of a Ni/Au/Ge eutectic (McGregor, 1996b), covering the entire back surface. The front surfaces were patterned with 5.6 mm diameter Ti/Au Schottky barrier contacts. A 1 µm thick film of 10B was then deposited over the Ti/Au contact. The devices were mounted to alumina chip holders with Ag epoxy and bonded

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Neutron Count Rate (Cts/Minute)

Counts per Channel

-10 Volts bias - 30 Volts bias - 50 Volts bias - 70 Volts bias - 90 Volts bias - 130 Volts bias

103

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101 0

200

400

600

800

1000

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1400

with Au wire. Afterwards, a very thin film (75 µm) of polyethylene was applied over the 10B coated surface. 2. NEUTRON MEASUREMENTS A semi-insulating bulk GaAs Schottky barrier detector was enclosed in a light impenetrable aluminum box to shield it from RF and visible light interference. The GaAs device was connected to a standard commercially available Ortec 142A charge sensitive preamplifier. The device was placed in a double diffracted neutron beam from A-port at the Ford Nuclear Reactor (FNR) at the University of Michigan. Measurements of 15 minute duration were taken with various reverse bias voltages applied to the device, ranging from 10 volts to 130 volts. The results in Fig. 5 clearly show spectra from the main reaction products emitted from the 10B(n,α)7Li reaction. The 1.47 MeV α-particle peak is apparent at all operating voltages, and a “plateau” is observed from the 7Li particles with biases of 50 volts or greater. It is clear that the device works at very low voltages, but at the expense of lower count rates. From Fig. 6, the count rate continued to improve as the reverse bias voltage was increased, indicating that the charge collection efficiency was improving. Hence, higher voltages allowed for the neutron induced signal to be elevated above the system noise and background gamma-ray signals. However, the improvement in charge collection, corresponding to an improvement in neutron detection efficiency, diminished at operating biases above 80 Volts. From Fig. 7, a bias of 80 Volts produces a situation in which the background gamma-ray count

2500 2000 1500 1000 500 0 0

Channel Number

Figure 5: Comparison of reaction product spectra as a function of applied reverse bias voltage to a 10B coated GaAs diode. The 1.47 MeV alpha particle peak and the 840 keV 7Li ion plateau become apparent at the higher bias voltages.

3000

20

40

60

80

100

120

Reverse Bias Voltage (Volts)

Figure 6: Neutron induced count rate as a function of bias voltage for the 10B-coated GaAs detector under test. Although the count rate continued to improve as the voltage was increased, the improvement became minimal at reverse biases above 80 volts. rate reduced below the neutron induced count rate at approximately 240 keV, thereby giving credence in the calculations of stipulating that the LLD be set at 300 keV. Hence, within the gamma-ray background at the FNR A-port, the device can be operated at 80 volts with adequate confidence that events above the calibrated channel for 300 keV (channel 225 in the present case) are in fact neutron-induced events. A 30 minute measurement of the double diffracted thermal neutron beam was then performed at an operating reverse bias of 80 volts. Afterwards the measurement was repeated twice, first with a 2 inch thick boron-filled high density polyethylene (B/HDP) shield placed between the beam port opening and the detector, and second with a 2.5 inch thick brick of lead as the shield. Figure 7 shows the results from the three measurements. The B/HDP serves as an efficient filter for thermal neutrons while having little effect on gamma-rays. The lead performs well as a gamma-ray filter and only partially scatters neutrons from the beam. From Fig. 7, the main features expected from the 10B(n,α)7Li reaction are clearly shown in the unfiltered spectrum, in which the 840 keV 7Li ion appears as a plateau and the 1.47 Mev alpha particle appears as a peak. The poor energy resolution is a consequence of charged-particle energy selfattenuation as they transit through the 10B film. The thermal neutrons are almost completely blocked with the B/HDP filter, and the signal shown is composed primarily of background gamma rays. Notice that the scale is logarithmic, showing that the device is relatively insensitive to gamma-ray background interference. The distinctive features expected from

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103

Bare Beam 3 mm Cadmium 2 Inches B/HDP 2 Inches Lead 4 Inches Lead

104

Counts per Channel

Counts per Channel

Bare Neutron Beam 2 Inches B/HDP 2.5 Inches Lead

Plateau from 840 keV 7Li Particles

Peak from 1.47 MeV α-Particles

102

101

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0

Channel Number

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1400

Channel Number

Figure 7: Comparison spectra with a 10B coated GaAs detector. Reverse bias = 80 volts. Shown are spectra with no filter, 2 inches of B/HDP, and 2.5 inches of lead. The main energy features of the charged-particle reaction products are very apparent in the lead filtered and unfiltered spectra.

Figure 8: Comparison spectra with a 10B coated GaAs detector. Reverse bias = 70 volts. Shown are spectra with no filter, 3mm of cadmium, 2 inches of B/HDP, 2 inches of lead, and 4 inches of lead. The main features of the charged-particle reaction products are apparent in the lead filtered and unfiltered spectra.

the 10B(n,α)7Li reaction products are very apparent for the lead filtered spectrum. Since even fewer gamma-rays will transmit through the 2.5 inch lead brick compared to the B/HDP filter, the spectrum serves to demonstrate that the bare spectrum is indeed from neutron induced charged-particle emissions. Measurements were then performed with the detector in a neutron beam from the FNR E-port. E-port is aligned directly with the corner of the FNR D2O moderator tank, hence the neutron flux and gamma-ray component are much higher than observed at A-port. The E-port beam has a measured neutron flux of approximately 106 n-cm2/s (Kobayashi, 1996; Matsubayashi, 1999) and the gamma-ray exposure rate at E-port is 1.1 R/hr. Studies were performed with the device irradiated in the unshielded neutron and gamma-ray field from E-port, and with various shielding materials, in which cadmium, B/HDP, and lead sheets were placed in between the E-port outlet and the detector, allowing for a distance of 1 m from the detector face to the shielding materials. Shown in Fig. 8 are the results of the device operated at a reverse bias of 70 volts with no shielding, 3 mm cadmium shielding, 2 inches of B/HDP shielding, 2 inches of lead shielding, and 4 inches of lead shielding. The higher gamma ray environment of the E-port beam increases the interaction rate in the GaAs device, which is evident from the cadmium and B/HDP filtered spectra. Yet, the gamma-ray discrimination ratio, with the LLD set at 700 keV, is still approximately 5.8:1, showing that the device serves as a

self-discriminating neutron detector. The primary reason for the difference can be seen in Fig. 8, in which the gamma-ray energy (channel number) demonstrates a near exponential decrease in count rate with increasing energy, an expected result for gamma-ray absorption in a material (Knoll, 2000). However, the observed energy distribution of the charged-particle reaction products from 10B(n,α)7Li reactions does not exponentially decrease with energy, primarily because the neutron interactions take place only at the detector surface and the observed energies are from absorption of the energetic charged-particle reaction products. In some cases, nearly all of the charged-particle full energy is absorbed in the GaAs detector, thereby causing the appearance of a peak in the spectrum. Hence, a large separation in the energy spectrum between the gamma-ray events and the neutron-induced events becomes apparent near channel 500, and continues to separate further with higher energies. Channel 500 corresponds to an energy of approximately 725 keV. Shielding the detector with lead further enhances the gammaray discrimination ratio, in which the gamma ray component is reduced by a ratio of approximately 2.5:1 over the neutron component for every two inches of lead. Hence, a two-inch lead shield increases the gamma-ray discrimination ratio from 5.8:1 up to approximately 14.5:1 for operation at the E-port, a considerable increase.

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CONCLUSIONS Preliminary measurements indicate that the detector design is capable of detecting thermal neutrons while naturally discriminating out a significant amount of background gammarays. It is clear that the device operates well and can detect reaction products emitted from neutrons that interact within the 10 B film. The devices use a technology that has thus far proven to be very radiation resistant (McGregor, 1996b), and it is expected that the harsh environment accompanying spent nuclear fuel measurements will not significantly affect the device performance. Efficiency calculations for 24 keV neutrons indicate low numbers, yet the addition of high density polyethylene films over the existing design may work to increase the overall detection efficiency of the devices without compromising spatial resolution (Klann, 2000). It will be important to provide adequate lead shielding to reduce the gamma-ray interaction rate during the fuel element measurements. Next generation detector designs will be composed of 0.5 mm x 1.0 mm pixel sizes with a dual in-line array configuration. A layer of Al2O3 will be deposited between the 10B film and the contact surface to prevent cross talk and shorting between the pixels. Boron films up to 2 µm in thickness will be explored. A total of 64 pixels will be on each array element, and the detector elements will be hooked in series to increase the overall array length. The arrays will be used to assist with the analysis of fuel burnup on spent fuel elements. ACKNOWLEDGMENTS The research has been conducted under DOE Nuclear Engineering Education and Research Grant DE-FG0798ID13633. REFERENCES Berwick, K., Brozel, M.R., Buttar, C.M., Cowperthwaite, M. and Hou, Y., 1993, “Imaging of High Field Regions in SI GaAs Particle Detectors,” Inst. Phys. Conf. Pro., Vol. 135, pp. 305310. Derhacobian, N., and Haegel, N.M., 1991, “Experimental Study of Transport in a Trap-Dominated Relaxation Semi-conductor,” Phys. Rev. B, Vol. 44, pp. 12754 -12760. Feigl, B., and Rauch, H., 1968, “Der Gd-Neutronenzähler,” Nucl. Instr. and Meth., Vol. 61, pp. 349-356. Foulon, F., Bergonzo, P., Brambilla, A., Jany, C., Guizard, B., and Marshall, R.D., 1998, “Neutron Detectors Made from Chemically Vapour Deposited Semiconductors,” Proc. MRS, Vol. 487, pp. 591-596.

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