Thermal Neutron Energy Research Articles

Measurement of Steady-State, Space-Dependent, Thermal-Neutron Spectra in Beryllium Assemblies with Strong Transverse Leakage

Thermal-neutron energy spectra have been measured as a function of transverse buckling and of space position along the longitudinal axes of 25.4- x 25.4- and 35.6- x 35.6-cm beryllium assemblies by the slow-chopper, time-of- flight technique using the thermal column of the Georgia Tech Research Reactor as the steady- state neutron source. For neutrons moving in the (forward) positive Z direction, no evidence was found of the establishment of, or the tendency toward, discrete asymptotic decay conditions from the strongly space-dependent spectra in either assembly. This is a direct experimental verification of the disappearance of the discrete set of eigenvalues and eigenfunctions in the diffusionlength problem with transverse leakage. These results are in at least qualitative agreement with the transport theory predictions of Williams, but in disagreement with the diffusion theory results of Ahmed, Kothari, and Kumar which predict that a true discrete mode should exist in beryllium assemblies as small as 30 x 30 cm. (auth)

Study of (n, α) reactions at thermal energies

Cross-section ratios σ(n, α)/σ(n, γ) at thermal neutron energies for the target nuclei 180Hf, 203Tl, 208Pb and 209Bi have been measured to be (38.5 ± 1.1) × 10 −3, ≦ 2.5 × 10 −3, (15.4 ± 1.7) × 10 −3 and (4.4 ± 0.4) × 10 −3 respectively. The activation technique was used for measuring these cross sections.

Terrestrial Age of Nineteen Stony Meteorites derived from their Radiocarbon Content

THE effects of cosmic radiation on meteorites have been studied thoroughly during the past 15 yr1,2. 14C is chiefly produced by three nuclear reactions: 14N(n,p)14C, 16O(p,3p)14C and 16O(n,2pn)14C. The first reaction occurs in the Earth's atmosphere3–4 and provides the basis for the well known radiocarbon dating method. The 14N(n,p)14C reaction has its largest cross-sections (σ=1.7 bar) in the thermal neutron energy region. The 16O spallation reactions (σ=2 mbar) require energies above 10 MeV. The air mass of 1 kg cm−2 above the surface of the Earth dissipates most of the energy of the primary cosmic rays. Therefore, 14C production in rocks on the Earth's surface is negligible. The surface of an object in space, however, is exposed to the full flux of the primary cosmic radiation. Meteorites, therefore, contain a measurable amount of high-energy spallation products. Meteorites contain very little nitrogen, and hence the 14C production is dominated by 16O spallation. The equilibrium 14C activity in stony meteorites per unit weight has a different value from that of the terrestrial atmosphere. The approximate average values for the 14C production rate are 120 14C atoms min−1 cm2 at the Earth's surface (containing 1 kg of air) and 60 14C atoms min−1 kg of meteorite5.

Proposed polarized-target-beam test for 3S 1- 3S 1 radiative transitions in thermal n-p capture

The development of techniques for producing polarized-proton targets and polarized-neutron beams may make it possible to set an upper limit on the probability of transitions from the 3S 1 states in the continuum of the low-energy n+p system to the 3S 1 part of the deuteron ground state. The absence of such transitions has been assumed in traditional theories of the thermal-energy neutron capture, as a consequence of the orthogonality of two eigenfunctions of a Schrödinger equation corresponding to two different energies. The applicability of this reasoning to the physical situation is discussed with the conclusion that it is not necessarily valid even apart from the slight effect of the admixture of the 3D 1 to the 3S 1 state. The density matrix of the colliding particles with arbitrarily assigned polarization vectors of beam and target is then used to calculate the angular distribution and degree of polarization of the capture gamma rays. It is found that if the whole of the approximately 9% “interaction effect” of Austern is to to be accounted for by 3S 1 → 3S 1 transitions, without any modification of the spin-flip transition intensity, asymmetries in the gamma-ray angular distribution ranging from 23% to 1.98% at a geometric-mean target-beam polarization of 0.954 to 0.577 respectively may be found. Larger or smaller effects may result depending on whether the spin-flip transitions are inhibited or enhanced by n-p interaction and structure effects. Appreciable effects on gamma-ray polarization are expected under conditions leading to appreciable gamma-ray angular distribution effects.

Calculation of the dose distribution about californium-252 needles in tissue.

HomeRadiologyVol. 98, No. 1 PreviousNext ArticlesCalculation of the Dose Distribution About Californium-252 Needles in TissueV. Rishnaswamy, John HaleV. Rishnaswamy, John HaleAuthor Affiliations Department of Radiology Hospital of the University of PennsylvaniaPhiladelphia, Penna. 19104V. RishnaswamyJohn HalePublished Online:Jan 1 1971https://doi.org/10.1148/98.1.155MoreSectionsPDF ToolsImage ViewerAdd to favoritesCiteTrack CitationsPermissionsReprints ShareShare onFacebookTwitterLinked In AbstractDose distribution around a point source of 252Cf in tissue due to neutron interaction has been evaluated by Monte Carlo methods. The dose due to neutron and gamma rays emitted by a 2 cm needle source was calculated and compared with published experimental data. The effect of absorber size on neutron dose was also determined.Article HistoryPublished in print: Jan 1971 FiguresReferencesRelatedDetailsCited ByGuidelines by the AAPM and GEC-ESTRO on the use of innovative brachytherapy devices and applications: Report of Task Group 167RavinderNath, Mark J.Rivard, Larry A.DeWerd, William A.Dezarn, H.Thompson Heaton, Geoffrey S.Ibbott, Ali S.Meigooni, ZoubirOuhib, Thomas W.Rusch, Frank-AndréSiebert, Jack L. M.Venselaar2016 | Medical Physics, Vol. 43, No. 6Part1Anniversary Paper: Past and current issues, and trends in brachytherapy physicsBruce R.Thomadsen, Jeffrey F.Williamson, Mark J.Rivard, Ali S.Meigooni2008 | Medical Physics, Vol. 35, No. 10Frontiers in Neutron Capture TherapyMark J.Rivard, MarkYudelev, FrankVan den Heuvel, Jacek G.Wierzbicki, Rodger C.Martin, Robert R.McMahon2001Measurements and calculations of thermal neutron fluence rate and neutron energy spectra resulting from moderation of 252Cf fast neutrons: Applications for neutron capture therapyMark J.Rivard2000 | Medical Physics, Vol. 27, No. 8Measurement of the dose components of fast and thermal neutrons and photons from a 0.1 mg 252Cf source in water for brachytherapy treatment planningS.Wanwilairat, R.Schmidt, T.Vilaithong, V.Lorvidhaya, W.Hoffmann2000 | Medical Physics, Vol. 27, No. 10Clinical brachytherapy with neutron emitting sources and adherence to AAPM TG‐43 dosimetry protocolMark J.Rivard, Jacek G.Wierzbicki, FrankVan den Heuvel, Rodger C.Martin, Robert R.McMahon1999 | Medical Physics, Vol. 26, No. 1Dosimetry for 252Cf neutron emitting brachytherapy sources: Protocol, measurements, and calculationsMark J.Rivard1999 | Medical Physics, Vol. 26, No. 8Californium-252 Isotope for 21st Century RadiotherapyA. S.Pavlov, K. N.Kostromina, M. A.Fadeyeva, A. L.Bocharov1997Californium-252 Isotope for 21st Century RadiotherapyA.Ralston, B. J.Allen1997Monte Carlo design study of a moderated source forin vivoneutron activation analysis of aluminiumD GLewis, S S ANatto, S J SRyde, C JEvans1997 | Physics in Medicine and Biology, Vol. 42, No. 4Clinical evaluation of252Cf neutron intracavitary therapy for primary endometrial adenocarcinomaYoshMaruyama, John R.Van Nagell, JustineYoneda, PaulDepriest, Richard J.Kryscio1993 | Cancer, Vol. 71, No. 12Progress in Neutron Capture Therapy for CancerJ. C.Yanch, R. G.Zamenhof, J.Wierzbicki, Y.Maruyama1992Biological Effects of Cf-252 Neutrons at Low Dose RatesJose M.Feola, Chester A.Nava, YoshMaruyama1982 | International Journal of Radiation Biology and Related Studies in Physics, Chemistry and Medicine, Vol. 41, No. 1Monte Carlo principles and applicationsD ERaeside1976 | Physics in Medicine and Biology, Vol. 21, No. 2Spectra and dosimetry related to neutron irradiations of the human bodyHIng, W GCross1975 | Physics in Medicine and Biology, Vol. 20, No. 6Status of dosimetry for 252Cf medical neutron sourcesL LAnderson1973 | Physics in Medicine and Biology, Vol. 18, No. 6Survival of murine leukaemia cells exposed to252Californium neutrons under conditions simulating implantation of these sources into tissue in radiotherapyRoger J.Berry, George D.Oliver, David L.George, JacquelineWerner, James T.Brennan1973 | The British Journal of Radiology, Vol. 46, No. 551Calculated depth dose tables for californium-252 sources in tissueVKrishnaswamy1972 | Physics in Medicine and Biology, Vol. 17, No. 1Dose Distributions around a Californium-252 NeedleR DColvett, H HRossi, VKrishnaswamy1972 | Physics in Medicine and Biology, Vol. 17, No. 3Calculated neutron dose rates for implantable californium-252 sourcesJ PWindham, AShapiro, J GKereiakes1972 | Physics in Medicine and Biology, Vol. 17, No. 4Proceedings of the British Institute of Radiology (Annual Congress)1971 | The British Journal of Radiology, Vol. 44, No. 527Recommended Articles RSNA Education Exhibits RSNA Case Collection Vol. 98, No. 1 Metrics Altmetric Score PDF download

A neutron detection system for the study of the (γ,n) reaction with a linear electron accelerator

A neutron detection system has been designed to measure photo-neutrons produced by a linear electron accelerator under an intense X-ray burst. It consists of BF 3 counters, a paraffin scatterer and some shieldings. The uniformity of the efficiency of the detection system is experimentally verified from thermal neutron energy up to about 5 MeV. The neutron yield curve near the threshold of 13C(γ,n) 12C reaction was observed with this detection system.

An empirical method for determining the relative efficiency of a Ge(Li) gamma-ray detector

An empirical method for determining the full-energy peak efficiency between 0.2 and 3.0 MeV and the two-escape peak efficiency between 2 and 9 MeV of a Ge(Li) gamma-ray detector is described. The full-energy peak efficiency is determined with the use of several nuclides which emit gamma rays with accurately known intensity ratios and a linear least-squares fitting procedure. With the full-energy peak efficiency determined, a number of high and low energy thermal neutron capture gamma ray cascades with known relative intensities are then utilized in the determination of the two-escape peak efficiency.

Slow neutron chopper-monochromator with curved slits

The design of two choppers with curved slits built for operation in the range of thermal and cold neutron energies is given. Chopper I can be used as mechanical selector in the whole range of thermal and cold neutron energies with a transmitted energy range ± 60%, chopper II gives a well-monochromatized (± 10%) neutron beam in the cold neutron energy range.

An investigation of the [formula omitted] reaction branching ratio

The branching ratio of the 10B(n, α) 7Li ∗, 7Li reaction was measured at thermal neutron energies by irradiating a 10B film and measuring the relative number of α-particles in the peaks associated with the ground state and the first excited level. Detection was by a silicon surface barrier detector. The percent of reactions going to the ground state was found to be 6.43±0.11.

Diffusion length of thermal neutrons in water between 24 and 82 C

The diffusion length of thermal neutrons in water has been measured at various temperatures between 24 and 82°C, using an 124Sb-Be (25 kev) source. The experiments were interpreted by a theoretical two-group model, one group referring to neutrons of thermal energy and the other group to neutrons of epithermal energy. The diffusion length L, corrected to unit density, and the absolute temperature T, were found to be related by the following expression: Lρ/ρ0 = constant × T0.487 ± 0.026

Calculation of Thermal Neutron Flux Spectra in a Polyethylene Moderated Medium

A model is proposed for calculating the scattering of thermal energy neutrons by polyethylene. It is seen that a reasonable fit to the experimental total cross section results. This model is then used to calculate infinite medium spectra and the results compared with calculations using water and perfect gas scattering kernels, with a harder predicted spectrum apparent. The results are compared with experimental data for two amounts of absorption, and agreement between experiment and theory using an appropriate scattering model is observed. The model is then used to calculate reactor quantities for a particular one-dimensional finite lattice. The sensitivity of the flux spectra and criticality to the choice of scattering kernel is presented. The former quantity is more sensitively dependent than the latter.

Study of theSm149(n, α)Nd146Reaction with Thermal Neutrons

The cross section for the ${\mathrm{Sm}}^{149}(n, \ensuremath{\alpha}){\mathrm{Nd}}^{146}$ reaction at thermal neutron energies was measured by observing the alpha-particle spectrum of natural Sm and isotopically enriched ${\mathrm{Sm}}^{149}$ targets in the thermal column of a reactor. Alpha-particle groups were observed at 8.72 and 9.12 MeV corresponding to transitions to the first excited state and ground state of ${\mathrm{Nd}}^{146}$. The 2200-m/sec cross sections (${\ensuremath{\sigma}}_{0}$) for the two groups were calculated to be 121\ifmmode\pm\else\textpm\fi{}15 mb and 22\ifmmode\pm\else\textpm\fi{}10 mb, respectively, from the experimental results. The 4- resonance at 0.0967 eV accounts for most of the cross section to the first excited state, but cannot contribute to the population of the ground state. It is postulated that the population of the ground state arises from a contribution of a bound 3- state in ${\mathrm{Sm}}^{150}$. Calculated values for the ($n, \ensuremath{\alpha}$) cross section are compared with the experimental results.

Removal of Higher Orders in the Thermal Region from a Neutron Crystal Spectrometer

A Be ``single crystal'' filter was used to eliminate higher-order contamination in the thermal neutron energy range from the monochromatic beam of a single crystal neutron spectrometer employing Ge(111) as monochromator. It is shown that 6.4-cm Be crystal filter is sufficient to suppress higher order contamination to less than 0.5% in the neutron energy range ∼0.02–0.06 ev and the attenuation of the primary beam is a factor of 2–3 in this energy range.

Nuclear Thermalization in Pulsed Moderators

A theoretical investigation was made into the timeenergy and space- energy behavior of a pulse of fast, fission energy neutrons introduced into non- multiplying, moderating mcdia. Interesting results were obtained for the thermal region where the neutron energy is less than one ev and the neutron-nucleus scattering process is essentially quantum mechanical. The spatial dependence of the thermal neutron energy spectrum and its departure frond the Maxwellian was investigated. A method for defining the region in a finite medium within which space and energy are accurately separable was established. The average energy as a function of position was also obtained. The behavior of a typical l/v scatterer, such as water, is illustrated. (P.C.H.)

Time-Dependent Thermal-Neutron Energy Spectra in a Monatomic Heavy Gas

The time-dependent thermal neutron energy spectra, for times greater thsn the slowing-down time, were generated in a monoatomic heavy gas using multigroup formalism. These spectra were obtained for infinite as well as finite media of Be and graphite. The behavior of asymptotic energy spectra during the last stage of neutron thermalization and diffusion periods was studied. The thermalization time constant for the establishment of the final Maxwellian velocity distribution of neutrons, in a monoatomic heavy gas, was estimated to be equal to (l.l74 xi SIGMA /sub so/v/sub o/)/sup -1/. Total thermalization times for neutrons in Be and graphite were found to be equal to 114 and 238 mu sec, respectively. Using the energy-dependent transport mean free path, the diffusion cooling coefficient for Be was calculated to be equal to 0.890 cm/sup 2/. For graphite, under the constant diffusion coefficient assumption, the diffusion cooling coefficient was determined to be equal to 1.922 cm/sup 2/. (auth)

Slow-Neutron Scattering Cross Sections of Terbium, Ytterbium, and Lutetium

The coherent neutron scattering cross sections of terbium, ytterbium, and lutetium as determined from the neutron diffraction measurements on Tb${\mathrm{C}}_{2}$, Yb${\mathrm{C}}_{2}$, Yb, and Lu metals are found to be 7.2\ifmmode\pm\else\textpm\fi{}0.4, 20.0\ifmmode\pm\else\textpm\fi{}0.4, and 6.70\ifmmode\pm\else\textpm\fi{}0.37 barns, respectively, all with amplitude of positive phase. The total scattering cross sections of Tb, Yb, and Lu and the magnetic scattering cross sections of ${\mathrm{Tb}}^{+3}$ and ${\mathrm{Yb}}^{+3}$ are evaluated for thermal neutron energies. The neutron diffraction data also show the Debye temperatures of Yb and Lu metals to be 109\ifmmode\pm\else\textpm\fi{}5\ifmmode^\circ\else\textdegree\fi{}K and 161\ifmmode\pm\else\textpm\fi{}7\ifmmode^\circ\else\textdegree\fi{}K, respectively.

Neutron evaporation and level densities in excited nuclei

The theory of a cascade neutron evaporation process is derived in detail in a form suited to the treatment of highly excited nuclei which have a continuous distribution of excitation energies following a primary direct interaction. The results are found to give good agreement with neutron spectra of nuclei following bombardment with 190 MeV protons observed by Gross. Data from medium energy neutron reactions, thermal energy neutron resonances, and heavy ion reaction experiments are compared. Formulae are derived in an appendix for the effect on the level density of a gap in the single particle level spacing at the Fermi level.

The Thermal Neutron Spectrum in a Heterogeneous Reactor

The thermal neutron energy spectrum in a homogeneous medium has been studied by many authors. In case of heterogeneous reactors, the absorption in fuels is so strong that the thermal neutron energy spectrum in the fuel will have a higher energy than in the moderator. The different temperatures of these media will also cause a difference of spectra. The thermal neutron energy spectra of each medium in a heterogeneous reactor are studied. The thermal neutron energy spectrum and the neutron temperature in light-water moderated and UO/sub 2/ fueled reactors were calculated by a numerical method and the approximate Wigner- Wilkins method. (auth)

Capture Cross Sections for Thermal Energy Neutrons. II

Cross sections for capture of slow neutrons by a number of elements were measured by determining the intensity of the $\ensuremath{\beta}$-activity induced under standard neutron irradiation. The number of neutrons was measured by using as standard the activity induced in Mn, the cross section for capture by Mn being taken from former experiments. Data are reported for neutron capture by ${\mathrm{Na}}^{23}$, ${\mathrm{Al}}^{27}$, ${\mathrm{P}}^{31}$, ${\mathrm{K}}^{41}$, ${\mathrm{V}}^{51}$, ${\mathrm{Cu}}^{63}$, ${\mathrm{Cu}}^{65}$, ${\mathrm{As}}^{75}$, ${\mathrm{Br}}^{79}$, ${\mathrm{Sb}}^{121}$, ${\mathrm{I}}^{127}$.

Capture Cross Sections for Thermal Energy Neutrons

Cross sections for capture of thermal energy neutrons were measured for several elements. The method consisted in comparing, by a B${\mathrm{F}}_{3}$ ionization chamber, the density of thermal neutrons in water and in a solution of the element investigated. In order to avoid corrections for geometry, the effect of the substance was compared with that of boron solutions of different concentrations, and consequently the measurements directly give the ratio of the cross sections of the elements investigated to the cross section of boron. Data are given for N, F, Na, P, Cl, K, V, Mn, Co, As, Se, Br, Cd, I, Ba.