Total Neutron Yield Research Articles

Anisotropy factors for the determination of total neutron yield from the D(d, n)He 3 and T(d, n)He 4 reactions

Tables have been computed of the anisotropy factor, R, which is useful in determining, by the associated particle method, the total neutron yield produced in the reactions T(d, n)He 4 and D(d, n)He 3. The range of incident deuteron energy covered is 25 to 500 keV, at associated particle colatitude angles of 90 to 180 deg. Thick- and thin-target cases have been treated. For the T(d, n)He 4 reaction, the total neutrons produced per alpha particle incident on the detector is given by the relation, N = (4 π/ ΔΦ) R. In the case of the D(d, n)He 3 reaction, it is more convenient to detect the protons produced in the competing reaction D(d, p)T. The total neutrons per proton incident on the detector is the given by N = ( 4π ΔΩ )R( Y d,n Y d,p ) , where (Y d,n/Y d,n) is the ratio of total neutrons to total protons produced. Tables of this ratio, of R, and of their product are given.

The problem of measuring the absolute yield of 14-MeV neutrons by means of an alpha counter

The assumptions used to derive the total neutron yield per detected alpha particle (from the D-T reaction) which were derived in an earlier report are re-examined in the light of additional experimental information. It is concluded that, for an alpha counter at 90° to the incident beam direction, the assumptions introduce practically no difficulties. Therefore, for precise monitoring in the absence of certain target information, it is recommended that this configuration be used. For counters at angles different from 90°, non-uniformity of target loading contributes the most serious error to the computed yield.

Total Neutron Yields from Light Elements under Proton and Alpha Bombardment

Total neutron production cross sections have been measured for targets of D, T, ${\mathrm{Li}}^{7}$, ${\mathrm{Be}}^{9}$, ${\mathrm{B}}^{11}$, ${\mathrm{C}}^{13}$, ${\mathrm{C}}^{14}$, and ${\mathrm{F}}^{19}$ under proton bombardment and of ${\mathrm{Li}}^{7}$, ${\mathrm{Be}}^{9}$, ${\mathrm{B}}^{10}$, and ${\mathrm{Si}}^{29}$ under alpha-particle bombardment. Energy ranges covered differ, but were contained between threshold and 5 Mev for protons and between threshold and 9 Mev for alphas. In several cases cross sections for the inverse reactions, ($n, p$) and ($n, \ensuremath{\alpha}$), have been computed and compared, where available, with direct measurements.

(γ, n) and (γ, 2n) reactions in93Nb

The (γ, n) cross-section of93Nb was measured by the residual activity method. Combining this result with the total neutron yield curve it was possible to separate the contributions of the (γ, n) and (γ, 2n) reactions. The ratio of these cross-sections was then compared with special attention to the 2.35 isomeric state (\(T_{\frac{1}{2}} = 13 hs\)) reported byJames; this activity was not found and we determined an upper limit for it as 0.02% of the 0.93 MeV line. An activity of 3.2% abundance due probably to γ rays of annihilation of positrons was found.

Fast photoneutrons from bismuth

The excitation functions for fast pliotoneutrons from bismuth are observed with (n, p) detectors. These indicate that the fast neutrons are produced predominantly by photons in the region of the giant resonance giving further experimental evidence that the giant resonance can be attributed to strong individual particle transitions rather than a collective type of photon absorption resulting in a general nuclear heating. The experimental angular distribution of the fast neutron component from the giant resonance can be represented by a value of B/A of 0.9 + 0.1 in agreement with that expected for shell model transitions. The relative number of fast neutrons was found to be about 7% of the total neutron yield. This is consistent with the idea that individual neutrons are excited from upper closed shell levels to levels in the continuum if one takes account of the interaction of the neutrons in these states with the rest of the nucleus, as in the model of Feshbach, Porter and Weisskopf.

Distribution of Prompt-Neutron Emission Probability for the Fission Fragments ofU233

The kinetic energies of both members of coincident fragment pairs were measured in a double back-to-back grid ionization chamber. The pulse heights were recorded only when coincident with a prompt fast neutron detected in either one of two neutron counters placed on opposite sides of the fission chamber. The strong angular correlation of the direction of motion of the prompt fission neutrons with the direction of motion of the emitting fragment permitted the identification of the latter as the emitting fragment. The frequency distributions of modes obtained by gating with a neutron from the light or from the heavy fragment were compared with the distribution recorded without a neutron coincidence. Comparison of the three sets of data show (1) that neutron emission from the light fragment predominates at low mass ratios, whereas at high mass ratios neutron emission from the heavy fragment is more probable; (2) that a broad maximum in the total neutron yield exists in the region of the most probable fission mode; and (3) that the average total kinetic energy curve is essentially the same for the three conditions of measurement.

GAMMA–NEUTRON REACTIONS IN O16 AND N14

The cross sections for the reactions O16(γ, n)O15 and N14(γ, n)N13 have been determined by measurement of the induced positron activities. The oxygen cross-section curve has a slowly increasing initial portion after which it rises rapidly to a maximum peak cross section of 11.4 millibarns at 24.0 Mev. The nitrogen cross-section curve has an initial peak at about 13 Mev. Above 17 Mev., this curve increases sharply to a maximum value of 2.84 millibarns at 24 Mev. In order to explain the shape of the nitrogen curve, a relative total neutron yield curve has been determined from 12 to 20 Mev. by detecting the emitted neutrons with a rhodium foil. This neutron yield curve has the same shape as the nitrogen saturated specific activity curve. The anomalous shapes of the nitrogen and oxygen cross-section curves are explained in terms of the variation of gamma-ray absorption with energy.

Thick Target Fast Neutron Yield from 15-Mev Deuteron and 30-Mev Alpha-Bombardment

The total yield and angular distribution of fast neutrons from bombardment of thick targets of pure Be, Al, Ti, Cr, Mn, Co, Cb, Mo, Ag, Cd, Ta, Au, Pb, and Bi by 15-Mev deuterons and 30-Mev alpha-particles was measured. These studies were made using the reaction ${\mathrm{S}}^{32}(n,p){\mathrm{P}}^{32}$ as a threshold detector and also with fission ionization chambers. The total neutron yield per second per \ensuremath{\mu}amp of the cyclotron beam, $N$, was observed to decrease with increase in nuclear charge, $Z$, of the target according to the approximate empirical equations, $logN=10.18\ensuremath{-}0.0234Z$ for deuteron bombardment and $logN=9.68\ensuremath{-}0.0234Z$ for alpha. In all cases studied, the angular distribution of fast neutrons from target bombarded by deuterons showed a pronounced peak in the yield in the forward direction.

Emission of Neutrons from Argon, Chlorine, Aluminum and Some Heavier Elements Under Alpha-Particle Bombardment

The emission of neutrons from chlorine, argon, scandium, titanium, manganese and iron under alpha-particle bombardment was established. The yield from argon is considerable and enabled a measurement of the energy of the neutrons to be made: the majority are associated with a group of energy change -5.6\ifmmode\pm\else\textpm\fi{}1.0 Mev, but two groups must be present. The excitation curve for these neutrons was plotted for alpha-particle energies between 3.5 and 9.0 Mev and varies smoothly in agreement with penetration through a barrier of radius 7.6\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$ cm. This smooth variation means that the total neutron yield does not change rapidly as a new group is excited, from which it is deduced that observations on single groups would show apparent resonance effects. The excitation curve for chlorine fits a theoretically derived function for a nuclear radius of 6.0\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$ cm and a similarly plotted curve for aluminum agrees with a radius of 5.8\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$ cm. These last elements have radii approximately fitting the formula $R={R}_{0}{A}^{\frac{1}{3}}$ with ${R}_{0}=1.94\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$, while argon has a radius which is definitely too large to fit the above relation. This abnormally large radius is linked with the large neutron content of the argon nucleus.