HIGH-SENSITIVITY SLOW NEUTRON COUNTER

For recording slow neutrons1, the reaction of their capture by \(B_5^{10}\) nuclei is used:

\[ B_5^{10} + n_0^1 \longrightarrow B_5^{11} \longrightarrow Li_3^7 + He_2^4, \]

which has a very large cross section \(\left(\sigma = 550 \cdot 10^{-24}\ \text{cm}^2\right.\) for thermal neutrons). In this process the ionization produced by \(\alpha\)-particles with an energy of \(1.6\ \text{MeV}\) is used.

Counting devices are usually proportional counters filled with a mixture of \(BF_3\) and argon; coatings of the chamber walls or of counters with substances containing boron are also used (boron carbide, borides). The efficiency of such counters reaches several percent.

The authors of the note being reviewed2 point out that, by using luminescent substances, it is possible to make a counter with an efficiency close to \(100\%\). Moreover, along with the use of luminescent boron compounds, it is possible to use a boron compound which, when introduced into the luminescent medium, would not cause quenching of the luminescence. It turns out that a number of complex ethers meet this requirement: methyl, ethyl, propyl, etc., borate added to an ordinary luminescent solution of phenylcyclohexane, triphenyl, and diphenylhexatriene. The addition of borates to these solutions is reflected only in a decrease in the magnitude of the pulse from the luminescence caused by particles. This occurs in proportion to the dilution of the solution, but the magnitude of the pulse can be restored by adding triphenyl.

The detecting device, as usual, consists of a photomultiplier illuminated by the luminescent substance.

The increase in the efficiency of ordinary boron counters is limited by the small thickness of the boron coatings on the walls (\(0.1\ \text{mm}\), because of the short range of \(\alpha\)-particles) and by the insufficient density of the \(BF_3\) gas filling the counter. Since the indicated luminescent liquids are transparent to their own radiation and have sufficient density, these limitations disappear.

For the work, a solution of equal volumes of methyl borate enriched in \(B_5^{10}\) and phenyl was used. Methyl borate was chosen because it contains the smallest number of extraneous atoms and has the highest density of ethers from the indicated series. This solution was used together with a 5819 photomultiplier and had the following characteristics: 1) the luminescence from an \(\alpha\)-particle knocked about 30 electrons out of the photomultiplier cathode; 2) the neutron lifetime in the medium was \(\sim 0.4\ \mu\text{s}\). In view of the short neutron lifetime, the presence of hydrogen in the luminescent medium does not interfere with the use of the counter for measuring the velocities of slow neutrons by time of flight.

When the counter is used in practice, in order to reduce noise it is necessary either to cool the photomultiplier with liquid air, or to connect in coincidence two photomultipliers illuminated by one luminescent solution. It is also necessary to prevent electrons with energies of \(100\ \text{keV}\) and higher from entering the detector.

V. S.

CITED LITERATURE

1. Beckler, Groshev, Isaev, Ionization Methods of Radiation Research, Gostekhizdat, 1950.
2. C. O. Muehlhause and G. E. Thomas, Phys. Rev., 85, 926 (1952).