Boron-based semiconductor solids as thermal neutron detectors Douglas S. McGregor 1 and Stan M. Vernon 2 1 S.M.A.R.T. Laboratory, Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109 2 Spire Corporation, 1 Patriots Park, Bedford, MA 01730 Abstract Commercially acquired pyrolytic boron nitride (BN) has been tested as a possible thermal neutron detector. Samples 1 mm thick with conductive contacts applied to both sides have been operated as traditional planar semiconductor detectors. The devices were tested in a double diffracted thermal neutron beam from a nuclear reactor. In general, the material has a high content of 10 B (19.9 % natural abundance), allowing for the excitation of charge carriers from the 10 B(n,α) 7 Li reaction. Being a solid, the material is attractive as a possible solid-state neutron detector. Experimental results from prototype devices are reported. Description Research work was performed to study the possibility of using boron-based semiconductor solids for thermal neutron detection. Materials studied included thin film CVD grown BP and BAs, as well as pyrolytic BN. The detection concept involves the use of the 10 B(n, α) 7 Li reaction, in which [1] 7 10 1 + + 0 7 Reaction Q Value Li(1.015 MeV) α (1.777 MeV), 2.792 MeV(to ground state) B n (1) Li*(0.840 MeV) + α (1.470 MeV), 2.310 MeV(1st excited state) Conceptually, the device is composed of a high resistivity boron-based semiconductor slab that is fabricated into a planar detector by fastening metallic electrodes on opposing surfaces. A neutron interaction within the device bulk releases the energetic particles of equation 1, thereby producing a cloud of electron-hole pairs. A
voltage applied across the device drifts the free charges to their respective electrodes, which in turn induce a measurable current (or voltage pulse) in an externally attached circuit. The basic configuration is depicted in Figure 1. Neutron Direction Boron Compound V Figure 1: The basic concept of the boron-based solid semiconductor neutron detector. Preliminary tests using CVD BP and BAs were largely unsuccessful. The CVD films were grown upon n-type Si substrates, which unfortunately had severe lattice mismatch. Further, the films formed into a polycrystalline, or more properly, microcrystalline film. Hence, the films were not single crystal, which meant that their charge transport properties were dismal. Further, only small samples could be handled and processed due to the severe bowing of the n-type Si substrates from the lattice mismatch, indicating that the boron films were also under strain. Boron is a p-type dopant in Si, hence growth of BAs or BP upon a n-type Si substrate can produce a pn junction diode at the BAs/Si (or BP/Si) interface, thereby producing a common boron-coated diode structure. Since it was the intention of the project to test the BAs and BP films as detectors, the entire device contact structure was fabricated upon the boron film rather than across the boron film and Si substrate. An interdigitated Ti/Au metal structure was evaporated on the boron films (see Figure 2). One side of the pattern was grounded and the other positively biased, and a charge sensitive preamplifier (Ortec 142A) was used to measure the signal from the positively biased leg. Several devices from numerous growth runs were tested, and various bias
schemes were implemented. Overall, tests results with the BP and BAs films from irradiation in thermal neutron beams from the Ford Nuclear Reactor failed to demonstrate any sensitivity to neutrons. There was some indication of photocurrent response to intense light, but the results were minimal and inconclusive. Cathode Anode Figure 2: The contact test pattern used to test the BP and BAs films. Pyrolytic boron nitride (PBN) materials were acquired from Advanced Ceramics to study its properties as a potential neutron detector. The samples acquired were 5mm x 5mm in area and 1 mm thick. Samples were prepared by cleaning them in a series of solvents, being TCE, acetone, isopropyl alcohol, and methanol, followed by a cascade deionized water bath. Each sample was blown dry with nitrogen. The samples were inserted into a multi-pocket shadow mask with 4.5 mm diameter circular patterns. The shadow mask and samples where then placed in an e-beam evaporator and metal contacts consisting of a 250-angstrom Ti layer followed by a 2000-angstrom layer of Au were patterned onto the samples through the shadow mask. The evaporation procedure was repeated for the opposing side, thereby producing a simplistic planar device. The PBN devices were mounted onto alumina substrates with Ag epoxy. Individual devices were connected into lightproof aluminum test boxes that were indexed to align the detectors in a thermal neutron beam from the Ford Nuclear Reactor [2]. The devices were tested with a double-diffracted neutron beam with an estimated thermal neutron flux of 2 x 10 4 n-cm -2 -s -1. The PBN device was operated at biases ranging up to 900 volts. Pulses were observed at voltages exceeding 800 volts and beyond. Most tests were performed with the bias stabilized at 900 volts.
The detector demonstrated discernable pulses that were determined to be neutron induced by observing the effect of a Cd shutter upon the count rate. With the Cd sheet blocking the neutron beam, the pulses were not observed. Without the Cd sheet, the pulses were again observed. The device appeared to yield symmetric results, in that reversing the voltage and detector direction still yielded a similar pulse height spectrum to the prior case. Counts per Channel 105 Inital response After 1 day in neutron beam 10 4 10 3 10 2 10 1 After one day of operation, the devices become much noisier and the LLD must be set higher. 10 0 0 50 100 150 200 250 Channel Number Figure 3: Pulse height spectra from a PBN neutron detector biased at 900 volts, showing the initial response and the degraded response after one day of operation in a thermal neutron flux of 2 x 10 4 n-cm -2 -s -1. There was, however, a very clear degradation in the observed pulse height spectrum over a short period of time, typically one day. The leakage current was observed to increase visibly within only one day for either bias direction. Further, the degradation was permanent. Shown in Figure 3 are thermal neutron induced spectra in a PBN device operated at 900 volts, in which the initial spectrum is shown, as is the spectrum after on hour of operation in the neutron beam. It should be noticed that the lower level discriminator (LLD) had to be increased to reduce dead time effects from the increasing leakage current. The increase in leakage current was observed for all PBN
devices tested. Presently, the damage mechanism has not been studied, although it is suspected that the charge particle emission products are creating damaged regions throughout the device that in turn allow for large leakage current conduction paths. References: 1. G.F. Knoll, Radiation Detection and Measurement, 3 rd Ed., (Wiley, New York, 2000). 2. H.K. Gersch, D.S. McGregor, and P.A. Simpson, A Study of the Effect of Incremental Gamma-Ray Doses and Incremental Neutron Fluences Upon the Performance of Self-Biased 10 B-Coated High-Purity Epitaxial GaAs Thermal Neutron Detectors, Nuclear Instruments and Methods, A489 (2002) pp. 85-98.