PHYSICS

M. M. KHALETSKII

MEASUREMENT OF TOTAL CROSS SECTIONS \(\sigma_t\) FOR NEUTRONS \(E_n=14.8\) MeV BY COUNTING \((n,\alpha)\)-COINCIDENCES\(^1\)

(Presented by Academician V. N. Kondrat’ev, 13 XI 1956)

Registration of monochromatic \(\alpha\)-particles from the reaction \(\mathrm{D}^2(\mathrm{T},\mathrm{n})\mathrm{He}^4\) makes it possible, with the aid of a coincidence circuit, to select in space a narrow beam of neutrons from this reaction. The use of a beam of neutrons correlated with \(\alpha\)-particles for measuring the cross sections \(\sigma_t\) makes it possible to eliminate the background (from other neutrons, in particular from neutrons of the reaction \(\mathrm{D}^2(\mathrm{D},\mathrm{n})\mathrm{He}^3\), and from other causes).

In the present work a thick T—Zr target was used (Fig. 1). A beam of magnetically separated deuterons with energy \(E_d=160\) keV was incident on the target. The \(\alpha\)-particles emitted from the target were registered by a NaJ(Tl) crystal of thickness \(d=0.5\) mm with an FEU-19. The window of the \(\alpha\)-counter subtended, with respect to the target, a solid angle \(\omega_\alpha=1.8\cdot10^{-3}\) steradian. On the other side of the target, at a distance of 75 cm from it, there was a neutron counter—a stilbene crystal 34 mm in diameter with an FEU-19.

Fig. 1. Scheme for measuring the cross sections \(\sigma_t\)

The neutron detector was set into the beam of “correlated” neutrons with an accuracy of \(\pm 0.5^\circ\); for this purpose the \(\alpha\)-counter was rotated through \(1^\circ\), first by the angle \(\varphi\) (in the horizontal plane), and then by the angle \(\theta\) (in the vertical plane), and at each position the \((n,\alpha)\)-coincidences of the \(n\)-counter were counted at one and the same \(\alpha\)-particle count (Fig. 2).

Fig. 2. Directionality of the beam of neutrons correlated with \(\alpha\)-particles. Stilbene neutron counter, \(d=34\) mm at a distance \(r=25\) cm from the target:
\(a\)—the counter was rotated about the target by the angles \(\varphi\);
\(b\)—by the angles \(\theta\)

Between the target and the neutron counter, at a distance of 26 cm from the target, cylindrical scatterers were placed into the neutron beam, and removed from the beam, by means of a special device. The dimensions of the scatterers were: diameter 25 mm, height 25 mm. The measurement of the total cross sections \(\sigma_t\) consisted in counting \((n,\alpha)\)-coincidences with the scatterer present and without it. From the attenuation of the \((n,\alpha)\)-coincidence count,

\[ T=e^{-n\sigma_t d}, \]

where \(n\) is the number of nuclei in

1 cm³, \(\sigma_t\) was determined. In the measurements a coincidence circuit with a resolution \(\tau = 5 \cdot 10^{-8}\) sec was used. The counting rate of coincidences in the measurements was \(\sim 30\) pulses/sec; simultaneously with the recording of coincidences, neutrons and \(\alpha\)-particles were counted channel by channel. The emergence of neutrons from the target was monitored by the \(\alpha\)-particle count.

Table 1

* According to the measurements of G. A. Ovsyannikov.

The results of the measurements of \(\sigma_t\) are shown in Table 1; the data of Coon et al. \(^{(2)}\) and Goodman \(^{(3)}\), obtained by another method for neutrons with energy \(E_n = 14\) MeV, are also given there.

The described method of measuring neutron cross sections can be extended to neutrons from the reaction \(\mathrm{D}^2(\mathrm{T}, n)\mathrm{He}^4\) for \(E_n \gg 14\) MeV and \(E_n < 14\) MeV (regardless of whether, at higher deuteron energies, the reaction \(\mathrm{D}^2(\mathrm{T}, n)\mathrm{He}^4\) remains a source of monochromatic neutrons \(^{(4,5)}\) or whether two neutron groups appear in it) and to neutrons from the reactions \(\mathrm{D}^2(\mathrm{D}, n)\mathrm{He}^3\) \(^{(1,6)}\) and \(\mathrm{p}(\mathrm{T}, n)\mathrm{He}^3\).

Institute of Chemical Physics
Academy of Sciences of the USSR

Received
2 XI 1956

REFERENCES

1. M. M. Khalepskii, Application of a coincidence circuit for measuring total cross sections of fast neutrons, Report of the Institute of Chemical Physics, Academy of Sciences of the USSR, 13 III 1953.
2. J. H. Coon; E. R. Graves, H. H. Barshall, Phys. Rev., 88, 562 (1952).
3. S. Goodman, Phys. Rev., 88, 686 (1952).
4. C. H. Johnson, A. Galonsky, Phys. Rev., 100, 1252 (A) (1955).
5. R. M. Eisberg, Phys. Rev., 102, 1104 (1956).
6. W. Franzen, P. Huber, L. Schellenberg, Zs. Naturforsch., 10a, 820 (1955).