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For the non-destructive investigation of irradiated in-core neutron detectors neutron-radiography has been applied. Both by direct and indirect methods a good resolution for critical detector parts was achieved. With these two methods irradiated self-powered neutron detectors with Co-, Er-, Hf- and Pt-emitters and a miniature fission chamber were investigated thus revealing the limits of conventional radiography compared to neutron-radiography when highly active objects are investigated.

Measurements of the mean prompt neutron lifetime have been made by a perturbation technique, by the use of a reactivity oscillator, and by the Rossi-a method in which the time correlation of fission chamber pulses is studied. The lifetime in Zeus was found to be 7 × 10 −8 sec and in the graphite reflected version of Zephyr, 6 × 10 −6 sec.

Measurements have been made of v, the number of prompt neutrons per fission, following neutron-induced fission of 240Pu. The measurements were made using a pulsed source of monoenergetic neutrons. The 240Pu was contained in a fission ion chamber placed close to the neutron source, and the neutrons detected in a 10 cm diameter plastic scintillator 60 cm from the fission chamber. A number of geometries were investigated experimentally verifying calculated corrections for neutron-fragment correlations etc. Values of v obtained were as follows: Neutron v energy 0·1 MeV 2·89±0·19 1·6 MeV 3·26±0·12

A method is proposed for the determination of the calibration factor P(t), used in fast fission ratio measurements. The method is based on the utilization of absolute data for the fast fission ratio δ28 in single rod. An experimental value of P(t), obtained by irradiating a single rod according to the present technique, is in agreement, inside the experimental errors, with the classical method utilizing a back-to-back fission chamber.

The signals of a Co- and a Hf selfpowered neutron detector were compared with the signal of a miniature in-core fission chamber during fast reactor power transients.

A method is described which permits the improvement of the signal-to-noise ratio in a system counting the α particles of a fission chamber, operating as a pulse chamber. This is achieved by picking-up two simultaneous signals of opposite polarity, obtained from the anode and the cathode, and adding them in a differential amplifier. The device which employs conventional fast current preamplifiers, permits counting of pulses of the order of 100 nA, that is, of the same order as those given by neutron capture in a non-proportional boron chamber.

Studies were made of the space and energy distributions of 3- to 15-Mev neutrons in boron carbide. Reactions H/sup 2/(H/sup 2/, n) He/sup 3/ and H/sup 2/ (H/sup 3/,n)He/sup 4/ were use d as sources for 3-Mev and 15-Mev neutrons, respectively. A proportional counter with BF/sub 3/ enriched up to 88%, a fission chamber with U/sub 235/ (enriched up to 75%) and Th/sup 232/, and threshold indicators of P/sup 31/(n,p)Si/sup 31/, Al/sup 27/(n,p)Mg/sup 27/ , 3/ (n,2n) Cu/sup 62/, and In/sup 115/(n, gamma ) I/sup 116m/ were used as neutron detectors. The results are plotted and tabulated. The ratio of the U/sup 235/ fission cross section sigma /sup 235/ (E/sub eff/) to sigma /sub B/) in (n, alpha ) reactions on B/sup 10/-boron carbide was determined. The ratio of 3-Mev neutron weakening in the region of fixed spectra was 0.97 plus or minus 0.03. The obtained relation was used for finding the effective energy E/sub eff/ = 120 plus or minus 10 kev. (R.V.J.)

Calculations were performed with the goal of designing a fission chamber in which the fraction of fragment pairs lost by foil absorption is independent of the fragment angular distribution.

Use of the time dependence of the fast neutron background is made to determine the efficiency of the fission chamber with 252 Cf. Spontaneous fission neutrons have been detected by the stilbene scintillator with pulse-shape discrimination circuitry. The efficiency of the chamber has been determined by the n-f coincidence method as well.