The All Boron Carbide Diode Neutron Detector: Experiment and Modeling Approach
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0981-JJ06-06
The All Boron Carbide Diode Neutron Detector: Experiment and Modeling Approach Ildar F. Sabirianov1, Robert W. Fairchild2, and Jennifer I. Brand3 1 College of Engineering/Nebraska Center for Materials and Nanoscience, University of Nebraska at Lincoln, 209N Walter Scott Engineering Center, 17th & Vine Streets, Lincoln, NE, 68588-0511 2 Physics, Nebraska Wesleyan University, 5000 St. Paul Ave. Olin Hall of Science 133, Lincoln, NE, 68504-2794 3 College of Engineering/Nebraska Center for Materials and Nanoscience, University of Nebraska at Lincoln, 17th & Vine Streets, WSEC, UNL, 245N, Lincoln, NE, 68588-0511 Abstract: Boron carbide diode detectors, fabricated from two different polytypes of semiconducting boron carbide, will detect neutrons in reasonable agreement with theoretical expectations. The performance of the all boron carbide neutron detector differs, as expected, from devices where a boron rich neutron capture layer is distinct from the diode charge collection region (i.e. a conversion layer solid state detector). Diodes were fabricated from natural abundance boron (20% 10B and 80% 11B.) directly on the metal substrates and metal contacts applied to the films as grown. The total boron depth was on the order of 2 microns. This is clearly not a conversion-layer configuration. The diodes were exposed to thermal neutrons generated from a paraffin moderated plutonium-beryllium source in moderated and unmoderated, as well as shielded and unshielded experimental configurations, where the expected energy peaks at at 2.31 MeV and 2.8 MeV were clearly observed, albeit with some incomplete charge collection typical of thinner diode structures. The results are compared with other boron based thin film detectors and literature models.
Keywords: diodes, neutron detection, icosahedra, closo-dicarbadodecaborane PACS numbers: 68.43.-h, 73.20.Hb, 31.15.Ct, 79.60.-i
Semiconducting boron carbide represents a new class of semiconducting materials with potential applications in neutron detection and radioactive decay calorimetry [1-7]. Neutron detectors with boron carbide have begun to attract attention [8-10]. Boron-10 has high cross section (approximately 3800 barns) for neutrons at lower energies (~25meV), based on the 10B(n,α)7Li neutron capture reactions:
B + n → 7Li (0.84MeV) + 4He (1.47MeV) + γ (0.48MeV)
10
94%
(1)
10
6%
(2)
B + n → 7Li (1.02MeV) + 4He (1.78MeV)
There has been considerable controversy as to whether an all semiconducting boron carbide diode can act as a neutron detector [7,9,11,12]. This controversy has been somewhat addressed by the fabrication of all boron carbide neutron detectors [4,6]. Such an all boron carbide solid state diode detector should exhibit a distinctly different pulse height spectrum than a device where the neutron capture is not an integral part of the diode: a device where a boron rich layer is instead placed adjacent to a diode (commonly called a conversion layer device). A number of conversion layer neutron detector diode devices have been fabricated [5,12-17
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