Tri-~-naphtylborine, a new scintillation material for thermal neutrons
The need for small thermal neutron detectors of high efficiency is obvious. Therefore much work concerning thermal neutron scintillators is carried out at several laboratories. At Oak Ridge for example 300 different materials have been tested recently in this respect 1).
An efficient thermal neutron scintillation counter should satisfy the following conditions.
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The density of atoms with a large capture cross section for thermal neutrons (B, Li or Cd) must be high.
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The material must be relatively insensitive for y-rays, as y-rays are nearly always present in neutron experiments. Therefore only light atoms should be present in the scintillator; organic fluorescent materials containing B or Li should be very favourable.
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The material must be transparant to its own fluorescent radiation. 4. The material must be available in transparent blocks or as a liquid. 5. The decay-time of the lightflash must be as short as possible.
In respect to these coiasfderations it seemed to be worth while to investigate the scintillation properties of tri-a-naphtylborine, B(CIoHy) 3 z). A small amount of this compound has been kindly prepared for us at the Laboratory for Organic Chemistry of the Leyden University *). When heated in vacuo this compound melted at 203°-205°C to a clear but yellowish liquid which solidifies on rapid cooling to a glass of the same colour. At temperatures over 50°C the glass proved to be unstable owing to recrystallisation. It seems very probable that the colour has to be ascribed to impurities. Tri-a-naphtylborine, both as a micro-crystalline powder as well as in the solid glass state, shows a blue fluorescence when exposed to ultra violet irradiation. The intensity of the fluorescent light is comparable to that of anthracene. \¥ith a small sample of 5 x 6 × 8 mm 3 preliminary measurements were carried out using the neutron flux emerging from a 100 mC Ra-Be source, kindly lent to us by the Institute for Nuclear Research (IKO) at Amsterdam. The very strong y-radiation was reduced by lead until its intensity was of the same order of magnitude as that of the thermal neutron flux.
The naphtylborine sample was sticked on to an uncooled 1 P 21 photomultiplier, the pulses of which, after being amplified by a J o r d a ri-B e 11 3) amplifier, were discriminated by an integral discriminator.
The counting rate was measured as a function of the pulse-height, the scintillator being irradiated either by the combined neutron-y-fluxes or by the y-flux only. In this way the fraction of the counting rate to be ascribed *) We are very much indebted to Prof. Havinga and Dr Koningsberger for the difficult preparation of this compound.