Neutron yield of the reactions Li6(t,n) and Li7(t,n)

The differential cross section is determined for the formation of neutrons at an angle of 0 ° in the reactions Li⁶(t,n) and Li⁷(t, n) in the 0.175–2.4 Mev triton energy range. In the reaction Li⁶(t,n) resonance in the neutron yield is observed at E_t = 1.875 Mev, corresponding to a Be⁹ excitation level of 18.936 Mev.Two resonances are detected in the neutron yield of the reaction Li⁷(t, n): at E_t = 0.765 Mev and E_t = 1.735 Mev, which indicates the presence of Be¹⁰ nucleus excitation levels of 17.78 and 18.46 Mev, respectively. The cross section of the reaction Li⁶(t, n) in resonance is d/d(0 °)_res = 37.2 mb/sr; for the reaction Li⁷ (t, n) the cross section at the first resonance is d/d(0 °)_res = 95 mb/sr; at the second resonance d/d(0 °)_res = 159 mb/sr.The angular distributions of neutrons are given for the interval of angles 0–135 ° (every 15 °) for triton energies of 0.358, 0.559, 1.006, 1.218, 1.370, 1.572, 2.123 Mev.The total cross section for the formation of neutrons at E_t = 2.123 Mev in the reaction Li⁶ (t,n) is equal to 324±32.3 mb in the reaction Li⁷ (t, n) to 1332±83.3 mb.     

The (p,n) reaction in lithium and the ground state of Be6

Using a time-of-flight method the neutron spectra in the Li⁶ + + p and Li⁷ + p reactions have been investigated at a proton energy of 9 Mev. Neutron groups have been found in the (p, n) reaction corresponding to the ground state in Be⁶ and the three lowest states of Be⁷ as well as a continuous neutron spectrum at lower energy, due to more complicated reactions. The observation of the neutron group for the Li⁶(p, n)Be⁶ reaction is the first experimental indication of the existence of the Be⁶ nucleus. The energy of the Li⁶(p, n)Be⁶ reaction is 5.2 Mev, the width of the ground state in Be⁶ is T < 0.3 Mev. The differential cross sections for neutron formation have been measured at 0, 15, 30, 60 and 120 °.     

The (p, n) reaction in lithium and the ground state of Be6

Using a time-of-flight method the neutron spectra in the Li⁶ + + p and Li⁷ + p reactions have been investigated at a proton energy of 9 Mev. Neutron groups have been found in the (p, n) reaction corresponding to the ground state in Be⁶ and the three lowest states of Be⁷ as well as a continuous neutron spectrum at lower energy, due to more complicated reactions. The observation of the neutron group for the Li⁶(p, n)Be⁶ reaction is the first experimental indication of the existence of the Be⁶ nucleus. The energy of the Li⁶(p, n)Be⁶ reaction is 5.2 Mev, the width of the ground state in Be⁶ is T < 0.3 Mev. The differential cross sections for neutron formation have been measured at 0, 15, 30, 60 and 120 °.The authors wish to thank their colleagues at the Cyclotron Laboratory, who made the present work possible; we are also indebted to A. I. Baz who participated in a discussion of the results.     

The measurement of the absolute intensity of neutron sources by a comparison with the reaction T(d,n)He4

A method for the measurement of the absolute intensity of neutron sources by a comparison with the neutron intensity from the reaction T(d, n)He⁴ is described. For the comparison of the intensities a detector was used which has an almost constant sensititity for a wide range of neutron energies. The detector consisted of a graphite prism in which, at a definite distance from the source (“constant”-sensitivity distance), the density of thermal neutrons was measured. The sensitivity of such a detector is constant, to an accuracy of 1–2 %, in the interval of neutron energies 0.1–8 Mev and is 13% lower for neutrons of energy 14 Mev. In 1953 this method was used for the measurement of the absolute intensity of a Ra-α-Be source with an accuracy of 4%.     

Measurement of thick-target gamma-ray production yields of the 7Li(p,p′)7Li and 7Li(p, γ)8Be reactions in the near-threshold energy region for the 7Li(p,n)7Be reaction

The gamma-ray production reactions, ⁷Li(p, p′)⁷Li and ⁷Li(p, γ)⁸Be, occur along with the neutron production reaction ⁷Li(p, n)⁷Be in a p-Li neutron source. These gamma-ray production reactions contribute to a patient's absorbed dose in boron neutron capture therapy (BNCT) when using a neutron beam from the ⁷Li(p, n)⁷Be reaction. The present work experimentally determined the thick-target gamma-ray production yields of the ⁷Li(p, p′)⁷Li and ⁷Li(p, γ)⁸Be reactions at incident proton energies of 1.670 and 1.870 MeV. The present results were compared with previous measurements. The gamma-ray production yield of ⁷Li(p, p′)⁷Li was measured to be 30%–50% smaller than as reported by previous studies. For the ⁷Li(p, γ)⁸Be reaction, the present thick-target yield is 30% smaller than one estimated from cross-section data measured in previous studies. The results must be included in future dose evaluation for BNCT using a p–Li neutron source.     

Measurements of Double-Differential Neutron Emission Cross Sections of 6Li, 7Li and 9Be for 11.5 MeV and 18.0 MeV Neutrons

Double-differential neutron emission cross sections (DDXs) of ⁶Li, ⁷Li and ⁹Be were measured for 18.0 MeV and 11.5 MeV incident neutrons produced by the T(d, n) and ¹⁵N(d, n) reactions respectively, using the Tohoku University Dynamitron time-of-flight (TOF) spectrometer. The data were obtained at 13 laboratory angles, and angular-differential cross sections (ADXs) of elastic and inelastic scattering neutrons were derived from the DDXs. For 11.5 MeV neutrons, we obtained the neutron emission spectra over the secondary neutron energies by newly employing the double TOF method as well as the conventional one. In the measurements at 18.0 MeV, we achieved better energy resolution than in our previous studies by using a neutron detector that has a larger solid angle and a thinner tritium target. The experimental results of DDXs and ADXs were compared with our previous results and the evaluated data given in JENDL-3.2, JENDL Fusion File and ENDF/B-VI. It is found that the JENDL data reproduce the experimental ones very well.     

Neutron production cross sections and energies for the reactions 7Li(p,n)7Be and 7Li(p,n)7Be1

Center-of-mass best values for the normalized Legendre coefficients and the 0° differential cross sections as functions of input energy have been derived from various experimental results for the reactions ⁷Li(p,n)⁷Be and ${}^7\text{Li}\text{(p,n)}{}^7\text{Be}{}^1$ (431 keV). This information has been used to calculate laboratory differential cross sections as functions of the laboratory proton energy and neutron emission angle which are given in tabular form together with the corresponding neutron energies.     

Measurement of the cross sections of the reactions 14N(n, α)11B and 14N(n, t)12C at neutron energies 5.45–7.2 MeV

The cross sections of the reactions ¹⁴N(n, α)¹¹B and ¹⁴N(n, t)¹²C have been measured for neutron energies 5.46–7.2 MeV. The neutrons are generated in the reaction D(d, n) on a solid titanium target. The work employs digital spectrometry. The charged-particle detector is a pulsed ionization chamber with a Frisch grid, filled with a kyrpton-nitrogen mixture. The cross sections are measured for four groups of α particles α₀, α₁, α₂, and α₃ from the reaction ¹⁴N(n, α)¹¹B and for tritium from the reaction ¹⁴N(n, t)¹²C. The energy resolution of the spectrometer was 60 keV. The errors in determining the cross sections for the reactions (n, α) and (n, t) are 10–15%. The measurement results are compared with the ENDF/V VI evaluation. Good agreement is obtained in the neutron energy range 5.45–6.5 MeV. At higher energies, the discrepancy reaches 30%. __________ Translated from Atomnaya énergiya, Vol. 101, No. 4, pp. 307–311, October, 2006.     

The measurement of the absolute intensity of neutron sources by a comparison with the reaction T(d, n)He4

A method for the measurement of the absolute intensity of neutron sources by a comparison with the neutron intensity from the reaction T(d, n)He⁴ is described. For the comparison of the intensities a detector was used which has an almost constant sensititity for a wide range of neutron energies. The detector consisted of a graphite prism in which, at a definite distance from the source (constant-sensitivity distance), the density of thermal neutrons was measured. The sensitivity of such a detector is constant, to an accuracy of 1–2 %, in the interval of neutron energies 0.1–8 Mev and is 13% lower for neutrons of energy 14 Mev. In 1953 this method was used for the measurement of the absolute intensity of a Ra--Be source with an accuracy of 4%.We consider it a pleasant duty to express our gratitude to G. N. Flerov for valuable advice and a constant interest in the work, to D. V. Timoshuk for discussion of the result and to S N. Solov'ev for constant help during the experiments.     

Angular distribution of elastic and inelastic scattering of 2.34 Mev neutrons from chromium,iron, and lead

Spectra are obtained for neutrons with initial energy E₀=2.34 Mev scattered by chromium, iron, and lead nuclei, and the corresponding differential cross sections for elastic and inelastic scattering are measured for angles from 30 ° to 135 °. The neutrons were obtained from the D (d, n) He³ reaction with an initial deuteron energy of 1 Mev and a neutron emission angle of 110 °. Nuclear photographic emulsion was used as the detectorspectrometer and as the monitor of the incident neutron flux. The scatterers were 2.8 cm diameter spheres. In calculating the cross sections for elastic and inelastic scattering, corrections for self-absorption and multiple scattering were introduced to the incident neutron flux in the scatterer. The angular distribution of inelastically scattered neutrons from chromium and iron, as well as from lead when the 0.53 Mev level was excited, was found to be isotropic within the limits of the experiment. Inelastic scattering from lead when the 0.805 and 0.890 Mev levels are excited cannot be considered isotropic.     

Tissue doses of fast and ultra-fast neutrons

An investigation has been carried out of depth doses in irradiation of tissue-like dummies (water, paraffin) with broad beams of fast and ultra-fast neutrons generated in the 1.5 meter cyclotron by bombarding a thick beryllium target with deuterons accelerated to 13 Mev (the reaction Be⁹ (d, n)B¹⁰) in the six-meter synchrocyclotron by bombarding a thick copper target with deuterons accelerated to 280 Mev (deuteron stripping reaction), and in the six-meter synchrocyclotron by bombarding a beryllium target with protons having an enervy of 480 Mev (proton charge-exchange reaction).The results of the investigation, when applied to the case of irradiation of tissue with narrow and broad beams of monoenergetic neutrons, permit conclusions to be formed concerning depth doses with any spectrum of neutrons, and make possible the calculation of the mean tissue dose that can be used in comparative radiobiology. Moreover, data on attenuation of the dose of neutrons of various energies in hydrogen- containing media may be of help to the engineer in the construction of shielding.Experimental and theoretical curves of the variation of the maximal tissue dose with hardness were obtained for broad and narrow neutron beams in the energy interval 0.1–500 Mev. These curves can be used for calibrating dosimeters.Views are expressed concerning methods of approach to evaluation of the biological effectiveness of nuclear radiations, and to establishing the maximum permissible dose for man.Tentative values are given for the maximum permissible neutron fluxes for man.     

Scattering of 14.1 MeV Neutrons by 6Li, 7Li, 9Be, 10B and 11B

Using a time-of-flight spectrometer, the differential cross sections were measured for the elastic and inelastic scattering of 14.1 MeV neutrons by ⁶Li, ⁷Li, ⁹Be, ¹⁰B and ¹¹B. In the case of elastic scattering by ⁷Li and ¹⁰B, correction was applied to subtract the contribution of inelastic scattering from the unresolved first excited state, after which, the elastic scattering data were compared with predictions based on the optical model. The potential parameters derived with a seven-parameter search yielded angular distributions agreeing with the present experimental data. The expressions for these parameters are presented as a function of mass number.

The experimental data on inelastic scattering were analyzed with the distorted wave Born approximation. The deformation parameters were estimated to be nearly equal to or larger than unity for these nuclei.     

Excitation Functions of Some Neutron Production Targets on (d,2n) Reactions

Deuterons are weakly bound nuclei which easily undergo break up reactions. This is well known at low energies, around 10 MeV, where deuteron beams have been used to produce neutrons very efficiently. These neutrons have a widely application field such as fission energy production and hybrid reactor systems. In this study, neutron emission cross sections produced by (d,2n) reactions for spallation neutron targets such as ⁵²Cr, ⁵⁶Fe, ^60,62,64Ni, ^63,65Cu, ^{182,183,184,186}W, ²³²Th, ²³⁵U and ²³⁸U have been investigated. Hybrid model and geometry dependent hybrid model were used to calculate the pre-equilibrium neutron-emission cross sections. For the reaction equilibrium process, Weisskopf–Ewing model calculations were selected. The obtained results have been discussed and compared with the available experimental data and found agreement with each other.     

Angular distribution of elastic and inelastic scattering of 2.34 Mev neutrons from chromium, iron, and lead

Spectra are obtained for neutrons with initial energy E₀=2.34 Mev scattered by chromium, iron, and lead nuclei, and the corresponding differential cross sections for elastic and inelastic scattering are measured for angles from 30 ° to 135 °. The neutrons were obtained from the D (d, n) He³ reaction with an initial deuteron energy of 1 Mev and a neutron emission angle of 110 °. Nuclear photographic emulsion was used as the detectorspectrometer and as the monitor of the incident neutron flux. The scatterers were 2.8 cm diameter spheres. In calculating the cross sections for elastic and inelastic scattering, corrections for self-absorption and multiple scattering were introduced to the incident neutron flux in the scatterer. The angular distribution of inelastically scattered neutrons from chromium and iron, as well as from lead when the 0.53 Mev level was excited, was found to be isotropic within the limits of the experiment. Inelastic scattering from lead when the 0.805 and 0.890 Mev levels are excited cannot be considered isotropic.This work was carried out during 1953–1955; the results were partially reported at the International Conference on the Peaceful Uses of Atomic Energy in Geneva, 1955.The author expresses his gratitude to Acting Member of the Academy of Sciences of the Ukrainian SSR A. I. Leipunsky for constant interest in the work, and to Candidates of Physical-Mathematical Sciences O. D. Kazachkovsky and I. I. Bondarenko for valuable discussion oi the results.     

0.04–4.0 Mev neutron fission cross section of Pu240

A detailed investigation of the Pu²⁴⁰ nucleus cross section is of interest with regard to an experimental verification of theoretical concepts of the energy dependence of fission probability as well as regarding possible uses of Pu²⁴⁰ as a nuclear fuel in fast neutron reactors.We measured the energy dependence of the fast neutron fission cross section of Pu²⁴⁰ for neutrons with the energy E_n = 0.04–4.0 Mev. The T(p, n)He³ reaction served as the neutron source. The Pu²⁴⁰ fission cross section in the plateau region (1–4 Mev) amounts to ~ 1.6 barn and is equal to only one-half of this value for the neutron energy E_n 0.7 Mev. A sharp decrease in the fission cross section value occurs as E_n decreases to 0.3 Mev; for a further decrease in E_n, the cross section value drops less sharply, and it remains practically constant (~ 0.065 barn) for 0.04 < E_n < 0.15 Mev. The correlation between the irregularities in fission cross section values and the levels of Pu²⁴⁰ nuclei which correspond to inelastic scattering channels is discussed.The authors extend their heartfeltthanks to A. I. Leiptmskii and I. I. Bondarenko for their helpfulness and interest in the work, to L. N. Usachev for the discussion of the results, to Yu. I. Baranov and N. E. Tokmantseva for their help in measurements, and to V. A. Romanov, G. A. Strigin, and Yu. I. Parfenov, who kept the accelerator in good running order.     

Development of the high-energy neutron fluence rate standard field in Japan with a peak energy of 45 MeV using the 7Li(p,n)7Be reaction at TIARA

In this study, we developed a 45 MeV neutron fluence rate standard of Japan. Quasi-monoenergetic neutrons with a peak energy of 45 MeV in the neutron standard field were produced by the ⁷Li(p,n)⁷Be reaction using a 50-MeV proton beam from an azimuthally varying field (AVF) cyclotron of the Takasaki Ion Accelerators for Advanced Radiation Application (TIARA). The neutron energy spectrum was measured using an organic liquid scintillation detector and a ⁶Li-glass scintillation detector by the time-of-flight method, and using a Bonner sphere spectrometer by the unfolding method. The absolute neutron fluence was determined using a proton recoil telescope (PRT) composed of the liquid scintillation detector and a Si(Li) detector that was newly developed in the present study. The detection efficiency of the PRT was obtained using the MCNPX code. The peak neutron production cross section for the ⁷Li(p,n)⁷Be reaction was also derived from the neutron fluence in order to confirm the neutron fluence of the TIARA high-energy neutron field. The peak neutron production cross section obtained in the present study was in good agreement with those of previous studies. The characteristics of the 45-MeV neutron field in TIARA were successfully evaluated in order to calibrate high-energy neutron detectors and high-energy neutron dosimeters.     

Production of 11C and 7 Be by TRIGA-II Research Reactor

¹¹C has been produced by irradiating for 30 min in the pneumatic tube of a TRIGA-III research reactor about 700 mg of metaboric acid (HBO₂), using the ^11B (p,n) ¹¹C reaction initiated by neutron-hydrogen recoil. The fast neutron flux (>lMeV) was 9×10¹¹n/cm²·sec. The ¹¹C produced was separated from the irradiated target by distillation of CO₂. The yield of ¹¹C at the end of irradiation was 0.27 μ.Ci.

⁷Be also was produced by irradiating for 15 hr 1 g of lithium hydroxide in the central experimental tube of the reactor, through the ⁷Li(p,n) ⁷Be reaction initiated by neutron-hydrogen recoil. The fast neutron flux was 2.2×10¹² n/cm²·sec. Separation of carrier-free ⁷Be from the irradiated target was obtained by co-precipitation with ferric hydroxide. The yield of 7Be was 0.2 μCi.

For determining the influence on the ⁷Be yield brought by differences in the chemical composition of target, irradiations were carried out on lithium compounds containing hydrogen, such as lithium hydroxide, lithium hydroxide hydrate, lithium acetate and lithium citrate. Among these four lithium compounds, lithium hydroxide provided the highest yield of ⁷Be per gram of target.     

Heavy hydrogen and neutron isotopes

The article discusses the latest experimental data on the states of four-nucleon and five-nucleon nuclei. They confirm the existence of three unbound excited states of He⁴: He⁴ (20.1 MeV, 0⁺, T = 0), He⁴ (22 MeV, 2^–, T = 0), He⁴ (24–25 MeV, 1^–, T = 1). The H⁴ and Li⁴ nuclei formed in the above processes have no bound states, and their lifetimes are of the order of 10^–22 sec. The H⁵ nucleus also has no bound state and decays to H³ + 2n with an energy of Q 1 MeV and a lifetime of the order of 10^–22 sec. The report that radioactivity has been observed in H⁵ is erroneous. An analysis of the binding energies of nuclei of known masses indicates that the tetraneutron also has no bound state. The H⁴, H⁵, and n⁴ nuclei, as well as the heavier hydrogen and neutron isotopes, lie beyond the limit of stability for decay with the emission of nucleons.Translated from Atomnaya Énergiya, Vol. 17, No. 1, pp. 3–9, July, 1964     

Neutron multiplication in 9Be: A stochastic approach

A method for the systematic derivation of the Multiplicity Spectrum (MS) of leakage neutrons from ⁹Be assemblies, starting from the forward Kolmogorov equation (familiar from stochastic neutron kinetics) is proposed. An exact solution to the multigroup Kolmogorov equation has been obtained using the method of Probability Generating Function (PGF). The effect of correlation between the two secondary neutrons in the (n, 2n) process on neutron multiplication has been investigated using this formalism. While the average multiplicity is not sensitive to correlation between the secondary neutrons in the (n, 2n) process, the MS for multiplicities ⩾ 5 is found to be highly sensitive. When correlation is accounted for the probability P(v) of leakage of v neutrons from a ⁹Be assembly differs by orders of magnitude from that without correlation for the higher multiplicities. As the correlation between the two neutrons contains valuable information, experimental study of the MS (especially the higher multiplicity events) can serve as a very powerful tool for the investigation of the (n, 2n) multiplicative process and can help resolve some of the observed discrepancies regarding the multiplication of 14 MeV neutrons in ⁹Be. The feasibility of the experimental determination of MS using a statistical correlation technique has been discussed in detail by Srinivasan (1985).     

Double-Differential Neutron Emission Cross Sections of 6Li and 7Li at Incident Neutron Energies of 4.2, 5.4, 6.0 and 14.2 MeV

Energy-angle double-differential neutron emission cross sections of lithium isotopes were measured at incident neutron energies of 4.2, 5.4 and 14.2 MeV for ⁶Li and of 5.4, 6.0 and 14.2 MeV for ⁷Li using a time-of-flight spectrometer. Care was taken in background subtraction and in data correction for sample-size effects. Detailed comparison of the present results was made with the evaluated data in JENDL-3PR1. A spectrum fitting method was used to extract the ^6,7Li(n, n'x)α and (n, 2n) reaction cross sections. Neutrons emitted from the (n, 2n) reactions were well described by the conventional evaporation model. A simple calculation with a final-state Coulomb interaction was effectively applied for the ^6,7Li(n, n'x)α reactions. Angle-integrated cross sections of the ⁷Li(n, n't)α reaction were in good agreement with the JENDL-3PR1 data except the data measured at 6.0MeV. The angular distributions of elastically and inelastically scattered neutrons were successfully analyzed with the coupled-channel method at the incident neutron energy of 14.2 MeV.