Neutron Capture In Hydrogen Research Articles

The effect of Compton scattering on gamma-ray spectra of the 2005 January 20 flare

Gamma-ray spectroscopy provides a wealth of information about accelerated particles in solar flares, as well as the ambient medium with which these energetic particles interact. The neutron capture line (2.223 MeV), the strongest in the solar gamma-ray spectrum, forms in the deep atmosphere. The energy of these photons can be reduced via Compton scattering. With the fully relativistic GEANT4 toolkit, we have carried out Monte Carlo simulations of the transport of a neutron capture line in solar flares, and applied them to the flare that occurred on 2005 January 20 (X7.1/2B), one of the most powerful gamma-ray flares observed by RHESSI during the 23rd solar cycle. By comparing the fitting results of different models with and without Compton scattering of the neutron capture line, we find that when including the Compton scattering for the neutron capture line, the observed gamma-ray spectrum can be reproduced by a population of accelerated particles with a very hard spectrum (s ≤ 2.3). The Compton effect of a 2.223 MeV line on the spectra is therefore proven to be significant, which influences the time evolution of the neutron capture line flux as well. The study also suggests that the mean vertical depth for neutron capture in hydrogen for this event is about 8 g cm−2.

Performance characteristics of the MIT fission converter based epithermal neutron beam

A pre-clinical characterization of the first fission converter based epithermal neutron beam (FCB) designed for boron neutron capture therapy (BNCT) has been performed. Calculated design parameters describing the physical performance of the aluminium and Teflon® filtered beam were confirmed from neutron fluence and absorbed dose rate measurements performed with activation foils and paired ionization chambers. The facility currently provides an epithermal neutron flux of 4.6 × 109 n cm−2 s−1 in-air at the patient position that makes it the most intense BNCT source in the world. This epithermal neutron flux is accompanied by very low specific photon and fast neutron absorbed doses of 3.5 ± 0.5 and 1.4 ± 0.2 × 10−13 Gy cm2, respectively. A therapeutic dose rate of 1.7 RBE Gy min−1 is achievable at the advantage depth of 97 mm when boronated phenylalanine (BPA) is used as the delivery agent, giving an average therapeutic ratio of 5.7. In clinical trials of normal tissue tolerance when using the FCB, the effective prescribed dose is due principally to neutron interactions with the nonselectively absorbed BPA present in brain. If an advanced compound is considered, the dose to brain would instead be predominately from the photon kerma induced by thermal neutron capture in hydrogen and advantage parameters of 0.88 Gy min−1, 121 mm and 10.8 would be realized for the therapeutic dose rate, advantage depth and therapeutic ratio, respectively. This study confirms the success of a new approach to producing a high intensity, high purity epithermal neutron source that attains near optimal physical performance and which is well suited to exploit the next generation of boron delivery agents.

On-line reconstruction of low boron concentrations by in vivoγ-ray spectroscopy for BNCT

Boron neutron capture therapy (BNCT) is a radiationtherapy in which the neutron capture reaction of 10B is used forthe selective destruction of tumours. At the High Flux Reactor (HFR) inPetten, a therapy facility with an epithermal neutron beam has been built. Inthe first instance, patients with brain tumours will be treated. The dosesdelivered to the tumour and to the healthy tissue depend on the thermalneutron fluence and on the boron concentrations in these regions. Anaccurate determination of the patient dose during therapy requiresknowledge of these time-dependent concentrations. For this reason, aγ-ray telescope system, together with a reconstructionformalism, have been developed. By using a γ-ray detector in atelescope configuration, boron neutron capture γ-rays of 478 keVemitted by a small specific region can be detected. The reconstructionformalism can calculate absolute boron concentrations using themeasured boron γ-ray detection rates. Besides the boronγ-rays, a large component of 2.2 MeV γ-rays emitted atthermal neutron capture in hydrogen is measured. Since the hydrogendistribution is almost homogeneous within the head, this component canserve as a measure of the total number of thermal neutrons in theobserved volume. By using the hydrogen γ-ray detection rate fornormalization of the boron concentration, the reconstruction tooleliminates the greater part of the influence of the inhomogeneity ofthe thermal neutron distribution. MCNP calculations are used as a toolfor the optimization of the detector configuration.Experiments on a head phantom with 5 ppm 10B in healthy tissueshowed that boron detection with a standard deviation of 3% requires aminimum measuring time of 2 min live time. From two position-dependentmeasurements, boron concentrations in two compartments(healthy tissue and tumour) can be determined. The reconstruction ofthe boron concentration in healthy tissue can be done with a standarddeviation of 6%. The γ-ray telescope can also be used forin vivo dosimetry.

Mixed field dosimetry of epithermal neutron beams for boron neutron capture therapy at the MITR‐II research reactor

During the past several years, there has been growing interest in Boron Neutron Capture Therapy (BNCT) using epithermal neutron beams. The dosimetry of these beams is challenging. The incident beam is comprised mostly of epithermal neutrons, but there is some contamination from photons and fast neutrons. Within the patient, the neutron spectrum changes rapidly as the incident epithermal neutrons scatter and thermalize, and a photon field is generated from neutron capture in hydrogen. In this paper, a method to determine the doses from thermal and fast neutrons, photons, and the B-10(n, alpha)Li-7 reaction is presented. The photon and fast neutron doses are measured with ionization chambers, in realistic phantoms, using the dual chamber technique. The thermal neutron flux is measured with gold foils using the cadmium difference technique, the thermal neutron and B-10 doses are determined by the kerma factor method. Representative results are presented for a unilateral irradiation of the head. Sources of error in the method as applied to BNCT dosimetry, and the uncertainties in the calculated doses are discussed.

Parameter estimation for hydrogen analysis by using transport method

A transport method is described which reduces greatly the number of calibration standards needed for hydrogen analysis by neutron induced prompt γ-rays. The counts in the photopeaks from neutron capture in hydrogen for various standard concentrations, the distribution of the source neutron rate entering the thermal group and the reaction rates in the samples are investigated theoretically using 100 energy group cross sections and experimental 252Cf spectra for a test configuration. Comparison of theoretical results with those measured from the test configuration shows good agreement.

The use of the reaction 9Be(α, γn) 12C to determine the energy scale and the efficiency at low energies of stellar collapse neutrino detectors

Experiments for the detection of gravitational stellar collapse ν e -events are most frequently based on the reaction: ν e + p → n + e + , followed by delayed neutron capture in hydrogen n + p → γ + d, E γ = 2.2 MeV. Data obtained by a low intensity Am/Be source (as n and γ-ray emitter), in association with MACRO scintillation counters, are being used to determine the capability of detecting delayed neutron capture from cascade n, γ-ray emissions. The 9Be( α, γn) 12C reaction is a valuable tool for setting the optimum “stellar gravitational collapse trigger” working conditions both for the detection of the primary ν e -events and for the detection of the delayed neutron captures. The use of the Am/Be source allowed several experimental checks of Monte Carlo calculated detection efficiencies.

Two-quantum radiative thermal neutron capture inH1

A value of (-5.2\ifmmode\pm\else\textpm\fi{}6.4) \ensuremath{\mu}b has been obtained for the cross section of the two-quantum radiative thermal neutron capture in hydrogen for the energy region $330<{E}_{\ensuremath{\gamma}1},{E}_{\ensuremath{\gamma}2}<1890$ keV. Restriction of the evaluation to a $\ensuremath{\gamma}$ energy range from 700 to 1520 keV yields (-0.4\ifmmode\pm\else\textpm\fi{}3.0) \ensuremath{\mu}b. The present results are weighted averages from several coincidence experiments performed with various types of detectors. In one experiment, using two NaI(Tl) detectors, special care was taken to cover also the region of lower $\ensuremath{\gamma}$-ray energies ($233<{E}_{\ensuremath{\gamma}1},{E}_{\ensuremath{\gamma}2}<2017$ keV), since here the most severe discrepancy to theoretical predictions is indicated by an earlier measurement. The existing data reject a hypothesis concerning nonorthogonality of the $^{3}S_{1}$ neutron-proton capturing and the deuteron ground state. Due to use of a ${\mathrm{H}}_{2}$O target from the present measurements at the same time the total cross section for the reaction $^{16}\mathrm{O}$($n$,$\ensuremath{\gamma}$)$^{17}\mathrm{O}$ could be determined as (187\ifmmode\pm\else\textpm\fi{}10) \ensuremath{\mu}b.NUCLEAR REACTIONS $^{1}\mathrm{H}$($n$,$\ensuremath{\gamma}\ensuremath{\gamma}$), $^{16}\mathrm{O}$($n$,$\ensuremath{\gamma}$), thermal $n$, measured ${\ensuremath{\sigma}}_{\ensuremath{\gamma}\ensuremath{\gamma}}$, ${\ensuremath{\sigma}}_{\ensuremath{\gamma}}$.

Upper limit for theH1(n,γγ)H2cross section

A value of (- 0.8 \ifmmode\pm\else\textpm\fi{} 2.5) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}5}$ has been measured for the ratio of double-photon to single-photon emission following neutron capture in hydrogen for $600 \mathrm{keV}<{E}_{\ensuremath{\gamma}}<1620 \mathrm{keV}$. The two Ge(Li) detectors used in the experiment subtended an angle of 85\ifmmode^\circ\else\textdegree\fi{} at the ${\mathrm{H}}_{2}$O target and were shielded from each other to reduce the background from the cross registration of single $\ensuremath{\gamma}$ rays. The upper limit of the measured two-photon cross section (-3 \ifmmode\pm\else\textpm\fi{} 8 \ensuremath{\mu}b) is two orders of magnitude larger than the most recent theoretical predictions ($\ensuremath{\approx}0.07$ \ensuremath{\mu}b) for this energy range.NUCLEAR REACTIONS $^{3}\mathrm{H}(n,\ensuremath{\gamma}\ensuremath{\gamma})$, thermal $n$, measured ${\ensuremath{\sigma}}_{2\ensuremath{\gamma}}$ Ge(Li) detectors.

Experiment on doubly radiative thermal neutron capture in hydrogen

An upper limit of 1.5×10−4 was measured for the branching ratio of the doublephoton to single-photon transition from the 2.2 MeV np capture state to the ground state of the deuteron. This result was obtained in a coincidence experiment with two Ge(Li) detectors. Based on the total cross section of 334.2 mb for the thermal neutron capture in hydrogen, the upper limit corresponds to a cross section of 49 μb for the two-photon emission in the energy region 46 keV<Eγ<2177 keV.

Search for Two-Photon Decay in ThermalnpCapture

An upper limit of ${10}^{\ensuremath{-}4}$ has been obtained for the branching ratio for the emission of two photons (${E}_{\ensuremath{\gamma}}&gt;0.57$ MeV) following thermal neutron capture in hydrogen. This limit was obtained with two Ge(Li) detectors shielded so as to reduce $\ensuremath{\gamma}$-ray scattering from one detector into the other and is a factor of 10 lower than has been recently reported.

Intensity estimation for a neutron beam for biological and medical applications

Fast neutrons of energies in the multi-MeV region are of great interest for biological and medical applications because of their special type of interaction with tissue-like material. The neutron energy is delivered to the tissue by one or several of the following means: (a) recoil protons, (b) recoil heavy particles, (c) nuclear disintegration, and (d) low-energy protons from neutron reactions with nitrogen and γ rays from neutron capture in hydrogen. This report discusses intensity estimations for different types of neutron beams, and is based on averaged and interpolated values from published and unpublished sources. In making the estimates, we have assumed realistic values for the distance from neutron source to exposure area. This assumption is necessary for two reasons: (a) to allow for the installation of sufficient shielding to reduce the background radiations in the irradiation area, and (b) to allow for neutron beam divergence so that reasonably large objects can be irradiated uniformly.

Etude dosimétrique d'un faisceau collimé de neutrons thermiques destine à l' analyse par radioactivation “in vivo”

In order to evaluate the absorbed dose in an individual locally irradiated by thermal neutrons with the intention of in vivo activation analysis, the study of the dosimetry of a collimated thermal neutron beam from the EL3 reactor at Saclay has been undertaken. Thermal flux measurements were done at several depths in tissue-equivalent phantoms representing a human neck as well as a human leg. According to these measurements, the absorbed dose due to the two main mechanisms of interaction of the thermal neutrons with the biological matter was evaluated theoretically as follows: (a) the absorbed dose due to the gamma rays from thermal neutron capture in Hydrogen and every other element in tissue was computed by the Mercure 3, IBM-360 FORTRAN IV, program. (b) the absorbed dose due to the 14N( n, p) 14C reaction was calculated by the use of the first collision dose per neutron per square centimeter formula. On the other hand, direct measurements of the absorbed dose were made with conventional ionisation chambers. The absorbed dose due to the gamma rays coming directly from the reactor core, as well as to the fast neutron fraction of the beam, was evaluated experimentally by straight measurement with an ionisation chamber of the dose rate behind a Li 6F shield. The dose equivalent thus evaluated was at the surface of the phantom for a 3 cm diameter thermal neutron beam and flux of 1.25 × 10 8 n cm −2 s −1 1. (a) Gamma capture dose: 180 mrem/min 2. (b) 14N( n, p) 14C reaction dose: 1190 mrem/min 3. (c) direct gamma and fast neutron dose: 94 mrem/min