The ${2}^{\ensuremath{-}}$ level in $^{8}\mathrm{Be}$ at the threshold for the reaction $^{7}\mathrm{Li}(p,n)^{7}\mathrm{Be}$ is shown to be a virtual state relative to threshold. A description of the state as an $S$-matrix pole in the scattering-length approximation gives a good account of its properties as observed in different reaction channels, and, in particular, removes an apparent contradiction about its width as observed through particle and $\ensuremath{\gamma}$-ray channels. The width of the ${2}^{\ensuremath{-}}$ state is found to be about 50 keV.

A grid-type ionization chamber filled with argon or an argon-carbon dioxide-mixture has been used to measure cross sections for the (n, α) and (n, p) reactions in Ar36 and the (n, α) reaction in Ar40 induced by mono-energetic neutrons with energies between 1.2 and 9 MeV. For reactions in Ar36, groups of alpha particles were observed leaving S33 in its ground and first seven excited states and groups of protons leaving Cl36 in its ground and first five excited states. The total cross section for these alpha groups increased from 2 mb at 1.2 MeV to about 160 mb at 4 MeV and then decreased to 61 mb at 8.7 MeV. The total cross section for the observed proton groups varied from 13 mb at 1.2 MeV to 290 mb at 4.7 MeV. For reactions in Ar40 groups of alpha particles were observed leaving S37 in its ground state and excited states at 0.65±0.06, 1.39±0.07 and 2.19±0.09 MeV. The total cross section for these groups varied from 0.1 mb at 6 MeV to about 3 mb at 9 MeV. The relative magnitudes of the cross sections for the (n, p) and (n, α) reactions are compared with the predictions of simple nuclear reaction theory.

Excitation functions at 0° and 90° and the total yield from the reaction C12(He3, n0)O14 have been measured for He3 bombarding energies from 1.8 to 5.5 MeV. Angular distributions were measured at 1.87, 2.33, 2.66, 2.87, 3.11, 3.93, 4.49, 4.75 and 5.19 MeV. The excitation curves and the total yield show considerable resonance structure. The angular distributions vary from isotropic to backward peaking at low energy and above 4 MeV bombarding energy are peaked in the forward direction.

The differential cross sections at several angles and some angular distributions of the following outgoing radiations induced by He3 in C12 targets from 1.8 to 5.4 MeV were measured; elastic He3, proton groups corresponding to the formation of the N14 ground state and excited states at 2.311, 3.945, 4.91, 5.10, 5.69, 5.83, 6.23, 6.44 and 7.03 MeV (p0 to p6, p8, p9 and p11), the 6.44 MeV gamma ray (γ6.44) following the p9 group, and the α groups corresponding to the formation of the C11 ground state and the first excited state at 2.00 MeV. Proton groups to N14 states at 6.05 and 6.70 MeV (p7 and p10) were not observed. Both the p9 and the corresponding γ6.44 show a strong isolated resonance at 2.990±0.010 MeV with Γlab = 125±10 keV. Analyses at this resonance on the elastic He3, p9, γ6.44, p1 and n0 data, assuming pure compound nucleus formation as the reaction mechanism, yielded Jπ = 52+ for the corresponding state in O15∗ at 14.46 MeV and even parity for the 6.44 MeV state of the residual nucleus N14. Indications of resonances at 2.45±0.04, 2.75±0.04, 3.28±0.04, 3.60±0.04, 4.20±0.01, 4.37±0.04, 4.6±0.1 and 5.0±0.1 MeV, corresponding to the states in O15 at 14.03, 14.27, 14.69, 14.95, 15.43, 15.57, 15.75 and 16.07 MeV were also observed.

The 15.1 MeV γ-ray excitation functions of the reactions B10(He3,pγ15.1)C12 (T = 1, 15.1 MeV state from 1.8 to 5.5 MeV, B11(d,nγ15.1)C12 from 1.5 to 5.5 MeV and C13(He3, αγ15.1)C12 from 2 to 5 MeV were measured. Resonances were observed in B10(He3,pγ15.1)C12 at 2.85±0.05 MeV (Γlab = 0.45±0.05 MeV) and 5.2±0.1 MeV (Γlab = 0.24±0.08 MeV); in B11(d,nγ15.1)C12 at 2.18±0.02 MeV (Γlab = 140±20 keV), 30.08±0.02 MeV (Γlab = 185 ±20 keV), 3.71±0.02 MeV (Γlab = 170±25 keV) and 4.4. MeV (broad); and in C13(He3, αγ15.1)C12 at 2.6, 3.6 and 3.9 MeV.

Measurements of neutron threshold energies by chargedparticle bombardment are considered. The determination of nuclear Q-values for ground and excited states is discussed. Methods for detecting threshold neutrons in the presence of large numbers of fast neutrons that leave the residual nucleus in ground or low-lying excited states are reviewed. The use of gamma -ray detection in measurements of neutron thresholds for excited states is also considered. Tables of neutron threshold energies for various (p,n), ( alpha ,n), (d,n), and (He/sup 3/,n) reactions are presented. (C.E.S.)

A magnetic-lens type beta-ray spectrometer has been used to study the radiations from the reaction N15(d, p) N16. Targets of CrN15 were made by induction-heating chromium foils to 1300°C in a N15-enriched gas atmosphere. Photoelectron spectra of γ-rays K-converted in Cd and Au radiators yielded γ-ray energies of 397.3±1.0, 296.2±1.0, 275.9±1.0, 120.1±0.5, and 100±3 keV. Excited levels of N16 at 397.3±1.0, 296.2±1.0 and 120.1±0.5 keV are inferred from the γ-ray energies. The K internal conversion coefficient of the 120 keV first-excited to ground state transition of N16 has been measured by the “external-internal” conversion method to be 7.23(1±0.20)×10−3. This value shows the 120 keV transition to be E2 which is consistent with the previously indicated spin and parity assignments of 2− and 0− for the ground level and first-excited level, respectively. The relative γ-ray branching of individual levels was also measured.

The disintegration of O16 and C12 by fast neutrons has been studied using a grid-type ionization chamber filled with CO2Ar or CO2Kr mixtures. Mono-energetic neutrons for studying the reactions from 5.0–8.8 MeV were produced with the D(d, n)He3 reaction using a gas target. The α-particle group from the O16(n, α)C13 reaction leaving C13 in its ground state was studied over the entire energy range. The excitation function showed 21 resonances corresponding to excited states in O17. The cross section varied from 2–206 mb. The α-particle group leaving C13 in its first excited state was studied from 7.6–8.7 MeV, the cross section increasing from about 3–12 mb. Unresolved α-particle groups leaving C13 in its second and third excited states were studied from 8.1–8.7 MeV, the cross section decreasing from an estimated 30–8 mb. The α-particle group from the C12(n,α)Be9 reaction leaving Be9 in its ground state was studied from 7.9–8.7 MeV. The cross section increased from 60–76 mb at 8.0 MeV and then decreased to 32 mb at 8.7 MeV.

The disintegration of O/sup 16/ and C/sup 12/ by fast neutrons was studied using a grid-type ionization chamber filled with CO/sub 2/-- Ar or CO/sub 2/-- Kr mixtures. Monoenergetic neutrons for studying the reactions from 5.0--8.8 Mev were produced with the D(d,n)He/sup 3/ reaction using a gas target. The alpha particle group from the O/sup 16/(n, alpha )C/sup 13/ reaction leaving C/sup 13/ in its ground state was studied over the entire energy range. The excitation function showed 21 resonances corresponding to excited states in O/sup 17/. The cross section varied from 2-206 mb. The alpha-particle group leaving C/sup 13/ in its 1st excited state was studied from 7.6--8.7 Mev, the cross section increasing from about 3- 12 mb. Unresolved alpha -particle groups leaving C /sup 13/ in its 2nd- and 3rd-excited states were studied from 8.1--8.7 Mev, the cross section decreasing from an estimated 30--8 mb. The alpha-particle group from the C/sup 12/(n, alpha )Be/sup 9/ reaction leaving Be/sup 9/ in its ground state was studied from 7.9-8.7 Mev. The cross section increased from 60--76 mb at 8.0 Mev and then decreased to 32 mb at 8.7 Mev. (auth)

The Si28(n, p)Al28, Si28(n, α)Mg25 and Si29(n, α)Mg26 reactions have been observed in a silicon solid state detector with a barrier depth equivalent to the range of a 7.5 MeV proton. A yield curve for the reactions has been measured between neutron energies of 4.6 and 8.5 MeV with an energy spread between 80 and 40 keV. At En = 8.5 MeV, transitions to two levels of Mg26, four of Mg25 and nine of Al28 have been observed. In the disintegration of Si28 strong resonant structure was observed, and below En = 6 MeV both reactions are strongly inhibited by the Coulomb barrier. Charged particle penetrability calculations predict the relative magnitudes of the Si28(n, α) and (n, p) reaction cross sections. A fast neutron spectrometer using a Si29 solid state counter is proposed.

Experiments carried out on the detection of slow neutrons from the ( alpha ,n) reaction in Be//sup 9/ show an increasing number of low-energy neutrons at 4.3 to 5.6 Mev. The energies and angular distribution of these neutrons for E alpha = 4.92 and 5.11 Mev were measured using the polyethylene spheremoderated neutron spectrometer. Results indicate a peaking in the forward direction and an energy distribution consistent with their origin from inelastic alpha scattering leaving Be/sup 9/ excited to the level at 1.75 Mev, which subsequently breaks up with neutron emission. At 5.11 Mev, the mean energy of these neutrons and the differential cross sections are: 0 deg (0.33 Mev, 22 mb/sr); 70 deg (0.21 Mev, 8 mb/ sr); and 120 deg (0.07 Mev, 2 mb/sr). The neutron spectrum of a Pu-Be source has a large number of low-energy neutrons having a peak intensity at 0.3 Mev. (auth)

Experiments carried out on the detection of slow neutrons from the ($\ensuremath{\alpha}, n$) reaction in ${\mathrm{Be}}^{9}$ show an increasing number of low-energy neutrons beginning at an alpha particle energy of 4.3 Mev, and extending to 5.6 Mev. The energies and angular distribution of these neutrons for ${E}_{\ensuremath{\alpha}}=4.92 \mathrm{and} 5.11$ Mev have been measured using the polyethylene sphere-moderated neutron spectrometer. The results indicate a peaking in the forward direction and an energy distribution consistent with their origin from inelastic alpha-particle scattering leaving ${\mathrm{Be}}^{9}$ excited to the level at 1.75 Mev, which subsequently breaks up with neutron emission. At an alpha-particle energy of 5.11 Mev, the mean energy of these neutrons and the differential cross sections are: 0\ifmmode^\circ\else\textdegree\fi{}(0.33 Mev, 22 mb/sr); 70\ifmmode^\circ\else\textdegree\fi{}(0.21 Mev, 8 mb/sr); and 120\ifmmode^\circ\else\textdegree\fi{}(0.07 Mev, 2 mb/sr). The neutron spectrum of a Pu-Be source has been determined to have a large number of low-energy neutrons with a peak intensity at 0.3 Mev.

The neutron spectra from the fission of U235, U233, and Pu239 by thermal neutrons have been measured by means of the sphere moderated neutron spectrometer. The spectra fit a Maxwellian distribution of the form N(E) = √ Ee−E|θ. The value of the energy parameter θ was found to be 1.332±0.030 MeV in the case of U235+n; the energies of the prompt neutrons from the fission of U233 and Pu239 were respectively 1.8 % and 4.0 % greater than that of U235. Experiments on the spontaneous fission of Pu240 and Cf252 give values of θ of 1.189 and 1.367 MeV. The most probable energy of the leakage neutrons from Jezebel was found to be 15 % lower than that of Pu239 + n. The variation of the energy of fission nneutrons with v is compared to evaporation theory.

The disintegration of B10 and F19 by fast neutrons has been studied using a grid-type ionization chamber filled with argon and boron trifluoride enriched in B10. Monoenergetic neutrons for studying the reactions from 0.2 to 8.2 MeV were produced with the Li7 (p, n)Be7, T(p, n)He3 and D(d, n)He3 reactions. Alpha groups from the B10((n, α)Li7 reaction leaving Li7 in its ground and first excited state at 478 keV were studied over the entire energy range. The cross section for the (n, α) reaction decreased rapidly from 1048 mb at 0.2 MeV to 180 mb at 1.2 MeV and then increased to 451±60 mb at the 1,84 MeV resonance. For higher energies broad resonance structure was observed with the cross section generally decreasing and reaching a value of 31 mb at 8.2 MeV. The B10(n, t2α) reaction was observed for neutron energies above 1.3 MeV. The cross section for this reaction increased to 80 mb at 4.5 MeV and was comparable to that for the B10(n, α)Li7 reaction above 5 MeV. The B10 (n, t2α) reaction was found to proceed mainly by three-body breakup or through the 4.63 MeV excited state of Li7. Some reactions may have proceeded through the 2.9 MeV level of Be8 but few, if any, proceeded through the ground state of Be8. The total excitation function for the F19(n, α)N16 reaction leaving N16 in the ground and first three excited states was studied for neutron energies above 2.9 MeV. Ten resonances were observed. The cross section increases to 135±50 mb at 5.80 MeV and then decreases at higher energies.

The nuclear reactions K39(n, p)A39 and K39(n, α)Cl36 have been studied by bombarding a KI(Tl) crystal with mono-energetic fast neutrons and observing scintillations due to emitted protons and α particles. Using good energy resolution (less than 10 keV) a yield curve has been obtained for the p0-group for neutron energies from 1.46 to 3.05 MeV, which exhibits 60 narrow resonances. Measurements with poorer energy resolution, varying between 70 and 150 keV, have been made in the neutron energy region from 2.3 to 8.7 MeV. From these absolute values of the reaction cross sections for the p0-group (En = 2.3 to 8.7 MeV), the α0-group (En = 2.3 to 3.8 MeV) and the p1-group (En = 2.5 to 3.5 MeV) have been extracted. Fourteen proton groups are seen in the pulse-height spectra, and from an analysis of the associated pulse-heights as a function of neutron energy, ten new excited states of A39 have been derived. The measured cross sections are compared with values obtained by other groups and with theoretical predictions.

(1961). Fast Neutron Physics Part I: Techniques. Nuclear Science and Engineering: Vol. 9, No. 4, pp. 523-523.