Photoneutron Threshold Research Articles

Measurement of Neutron Dose Equivalent within and Outside of a LINAC Treatment Vault Using a Neutron Survey Meter

This work concerns neutron doses associated with the use of a Siemens Primus M5497 electron accelerator, which is operated in the photon mode at 15 MV. The conditions offer a situation within which a fraction of the bremsstrahlung emission energies exceed the photoneutron threshold. For different field sizes, an investigation has been made of neutron dose equivalent values at various measurement locations, including: (i) At the treatment table, at a source-surface distance of 100 cm; (ii) at the level of the floor directly adjacent to the treatment table; and (iii) in the control room and patient waiting area. The evaluated neutron dose equivalent was found to range from 0.0001 to 8.6 mSv/h, notably with the greatest value at the level of the floor directly adjacent to the treatment couch (8.6 mSv/h) exceeding the greatest value on the treatment table (5.5 mSv/h). Low values ranging from unobservable to between 0.0001 to 0.0002 mSv/h neutron dose were recorded around the control room and patient waiting area. For measurements on the floor, the study showed the dose equivalent to be greatest with the jaws closed. These data, most particularly concerning neutron distribution within the treatment room, are of great importance in making steps towards improving patient safety via the provision of protective measures.

Analysis of Photo Neutron Production From the Patient During Radiotherapy Treatment using CR 39 SSNTD

Most radiation treatments for curing cancer are completely non invasive. Usually patients will undergo radiation treatment to relieve symptoms and pain. The research focuses the various processes involved in generating photo neutron spectrum and dose proceeding from accelerator for treatment. The electrons are stimulated to high energies through a linear tube. These high-energy electron beams similarly treats shallow malignant growths. It tends to strike an objective to create x-ray beams for deep-seated tumor treatments. Hydrogen thyratrons are the direct current power supply for the modulator that has a pulse-forming network with switch tube. This results in a flat-topped high voltage pulse from the modulator area to DC pulse for a few microseconds of the period.Photo neutron spectrum and dose calculated. 15 MV lead to photo neutron production from many of the elements including Tungsten, Lead, Tantalum Nitrogen, Phosphorus and so on. Particularly 207Pb has photo neutron threshold as low as 6.74 MeV and 182W has threshold 8.04 MeV. Moreover, Nitrogen and Phosphorus have a lesser threshold for photo-proton emission at 7.55 MeV and 7.3 MeV respectively. Considering the radiation-weighting factor for neutrons, it is obvious that, the neutron dose considered in the treatment planning process, and we have to incorporate some mechanism in the radiotherapy treatment delivery for neutron free photons.

Computational study of radiation doses at UNLV accelerator facility

A Varian K15 electron linear accelerator (linac) has been considered for installation at University of Nevada, Las Vegas (UNLV). Before experiments can be performed, it is necessary to evaluate the photon and neutron spectra as generated by the linac, as well as the resulting dose rates within the accelerator facility. A computational study using MCNPX was performed to characterize the source terms for the bremsstrahlung converter. The 15 MeV electron beam available in the linac is above the photoneutron threshold energy for several materials in the linac assembly, and as a result, neutrons must be accounted for. The angular and energy distributions for bremsstrahlung flux generated by the interaction of the 15 MeV electron beam with the linac target were determined. This source term was used in conjunction with the K15 collimators to determine the dose rates within the facility.

Present status of the low energy linac-based slow positron beam and positronium spectrometer in Saclay

A new slow positron beamline featuring a large acceptance positronium lifetime spectrometer has been constructed and tested at the linac-based slow positron source at IRFU CEA Saclay, France. The new instrument will be used in the development of a dense positronium target cloud for the GBAR experiment. The GBAR project aims at precise measurement of the gravitational acceleration of antihydrogen in the gravitational field of the Earth. Beyond application in fundamental science, the positron spectrometer will be used in materials research, for testing thin porous films and layers by means of positronium annihilation. The slow positron beamline is being used as a test bench to develop further instrumentation for positron annihilation spectroscopy (Ps time-of-flight, pulsed positron beam). The positron source is built on a low energy linear electron accelerator (linac). The 4.3 MeV electron energy used is well below the photoneutron threshold, making the source a genuine on-off device, without remaining radioactivity. The spectrometer features large BGO (Bismuth Germanate) scintillator detectors, with sufficiently large acceptance to detect all ortho-positronium annihilation lifetime components (annihilation in vacuum and in nanopores).

Integrated cross sections for excitation of nuclear isomers by inelastic photon scattering at giant resonance

In the view of the evidences arising from our experimental and theoretical studies, the long-standing picture of a two-humped excitation function for photoexcitation of isomers cannot be confirmed. Whereas the first maximum (at the photoneutron threshold) of the cross section of nuclear photon scattering can be attributed to inelastic (compound) scattering, the second large peak at about giant dipole resonance is mostly due to the elastic (direct) process. A second large peak or increase reported to appear in isomer production has been shown to be practically vanishing. On realizing such a situation, calculated estimates have been given for saturated integral cross section values for isomer activation, based on photoabsorption cross sections taken from the usual Lorentzian parametrization up to the photoneutron threshold. Results compare reasonably well to available experimental data acquired by gamma-ray spectrometry in a large set of stable nuclides having long-lived isomeric states.

Photofission in uranium by nuclear reaction gamma-rays

The ideal photon source for active interrogation of fissile materials would use monoenergetic photons to minimize radiation dose to surroundings. The photon energy would be high enough to produce relatively large photofission signals, but below the photoneutron threshold for common cargo materials in order to reduce background levels. To develop such a source, we are investigating the use of low-energy, proton-induced nuclear reactions to generate monochromatic, MeV-energy gamma-rays. Of particular interest are the nuclear resonances at 163keV for the 11B(p,γ)12C reaction producing 11.7MeV gamma-rays, 340keV for the 19F(p,αγ)16O reaction producing 6.13MeV photons, and 441keV for the 7Li(p,γ)8Be reaction producing 14.8 and 17.7MeV photons. A 700keV Van de Graaff ion accelerator was used to test several potential (p,γ) materials and the gamma-ray yields from these targets were measured with a 5″×5″ NaI detector. A pulsed proton beam from the accelerator was used to induce prompt (neutron) and delayed (neutron and gamma-ray) photofission signals in uranium which were measured with 3He and NaI detectors. We show that the accelerator data is in good agreement with Monte Carlo radiation transport calculations and published results.

Bremsstrahlung and photo-neutron yields from 5–25 MeV electrons on thin slabs of Al, Fe, Cu and Pb

In recent years, there has been a rapid growth in the use of low and medium energy electron accelerators for industrial and medical applications. Facilities of greater beam power for bulk processing are also under consideration. In planning these facilities, data on bremsstrahlung and photo-neutron yields from natural element targets of various thicknesses by electrons up to 25 MeV are required for target and shielding designs. In this paper, bremsstrahlung yields are calculated by the Monte-Carlo method using the code ETRAN. Neutron yields are calculated with the track length distributions from ETRAN and compiled neutron cross sections. The use of the Monte-Carlo method and evaluated neutron cross sections provides more reliable neutron yields, particularly at energies close to the photo-neutron thresholds. Data on structural materials including Al, Fe, Cu and Pb are presented.

Nuclear photoexcitation and delbr�ck scattering studied in the energy range 2?8 MeV

Elastic scattering by nuclei in the range of mass numbers between 64 and 238 has been studied with monochromatic photons in the energy range between 2 and 8 MeV. These photons were provided either by a Ti(n,γ) source installed in the tangential through channel of the Grenoble high flux reactor, or by24Na and56Co sources produced by deuteron bombardment of Al or Fe at the Gottingen cyclotron. The photoexcitation of 23 nuclear levels has been observed and the decay properties and groundstate widths of the majority of these levels have been determined. For the lead scattering target the coherent elastic differential cross section has been studied in detail. There is evidence that below the photo-neutron threshold the elastic scattering via virtual photoexcitation of the nucleus can be approximated by extrapolating the real part of the Giant Dipole Resonance amplitude along a Lorentzian curve. Coulomb corrections to Delbruck scattering seem to play a small role at 6.5 MeV.

Photoneutron production in detectors in high energy X-ray beams

Photoneutron reactions occur in detectors placed in high energy X-ray beams provided the beam energy is greater than the photoneutron threshold of the detector materials. Order of magnitude calculations of photoneutron yields per gram were made for silicon, oxygen, aluminium and carbon irradiated by bremsstrahlung beams with energies of 15, 18, 20, 24 MeV respectively. Using these calculations the photonuclear production in two detectors (NE 213 and 8″ polyethylene spherical rem-meter) were calculated. The significance of the results for the dosimetry of mixed neutron and high energy photon fields is discussed.

Characteristics of Composite Carbon Resistors Used for Measuring Accelerator Terminal Voltages

A voltage divider board composed of 1152, 22 MΩ composite carbon resistors used to measure the terminal voltage of a 3 MeV Dynamitron was investigated. The board was calibrated in situ using the 9Be and D photoneutron threshold reactions and a magnetic analyzer. The present calibration of the divider board which was made after over 5 years of extensive accelerator usage agrees within 2% with the original factory calibration. The linear temperature coefficient of resistance was measured to be +(0.85±0.10)×10−3/°C in the range 25 to 35°C. The divider board resistance decreased linearly with voltage (−1.1×10−5%/V) up to 2 MeV which corresponds to 1750 V on each resistor. The results show that composite carbon resistors and the techniques described can be applied with confidence to the measurement of high voltage.

A high efficiency graphite-moderated neutron counter

A high-efficiency neutron counter has been built, consisting of 40 1.5-m 10BF 3 proportional counters imbedded in a graphite cylinder 1.83 m long and 1.53 m in dia. The system is surrounded by 0.6 m of water on all sides to act as a reflector and shield, and is operated with the neutron sources placed at the center. The basic design was dictated by the requirements for an energy independent, high neutron efficiency for weak neutron sources in the presence of high γ-ray activity. This ruled out moderators having a high neutron absorption cross section or low photo-neutron threshold. The configuration design was based upon diffusion studies and extensive Monte-Carlo calculations. The resulting configuration consisted of a hexagonal matrix of individual counters forming five rings parallel to the cylindrical axis. The measured total efficiency was found to be about 60% and nearly independent of energy, although the relative efficiency among rings was energy dependent.

Threshold Photoneutron Cross Section forFe56

Individual levels of the compound nucleus ${\mathrm{Fe}}^{56}$ at an excitation energy of 11.2 MeV have been studied by examining the ($\ensuremath{\gamma},n$) cross section for natural iron from 0.7 to 70 keV above threshold with high resolution. The technique is generally applicable to the high-resolution study of photoneutron cross sections for photons whose energy exceeds the photoneutron threshold by an amount of the order of keV. Resonance parameters were obtained for certain favorable cases. The $\ensuremath{\gamma}$-ray strength function $〈\frac{{\ensuremath{\Gamma}}_{\ensuremath{\gamma}0}}{D}〉$ for electric dipole absorption in ${\mathrm{Fe}}^{56}$ was found to be 3.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$, which is a factor of 2 lower than the estimates obtained either from the low-energy wing of the giant dipole resonance or from the single-particle model of the nucleus.

System for energy control and absolute calibration of a 23 MeV betatron

The theoretical and technical features of a system used to determine and control the operating energy of a 23 MeV betatron are described. By observing the variation in orbit radius as a function of magnet excitation, we are able to predict the shape of the calibration curve to maximum operating energy. Although an absolute calibration of the predicted energy scale is possible, the present calibration is empirical and based on only the 16.22 MeV resonance in the O 16 (γ,n)O 15 reaction. Photoneutron threshold energies based on the present energy scale are in good agreement with accepted values.

Systematics of Neutron Separation Energies

Photoneutron thresholds for 73 isotopes have been measured by radioactivity and neutron detection methods using a 25-Mev betatron. The neutron separation energies inferred from the observed thresholds are in general agreement with the values predicted from mass data and reaction energies. Several discrepancies are observed between threshold and neutron binding energies where ground state transitions require a spin change $>~\frac{7}{2}$. For these nuclei, the threshold energies are consistent with neutron emission leaving the residual nucleus in an excited state.

BETATRON ENERGY CALIBRATION

A new betatron energy controlling circuit is described which has many advantages over those previously used. It is shown that there is a linear relationship between the helipot control on this circuit and the momentum of the electrons in the betatron just prior to striking the X-ray producing target. It is also shown that photoneutron thresholds can be used to calibrate the betatron energy scale with good internal consistency.

Photoneutron Thresholds for Iron, Germanium, Rubidium, and Hafnium

Photoneutron thresholds for ${\mathrm{Fe}}^{56}$, ${\mathrm{Fe}}^{57}$, ${\mathrm{Ge}}^{73}$, ${\mathrm{Rb}}^{85}$, ${\mathrm{Rb}}^{87}$, ${\mathrm{Hf}}^{177}$, and ${\mathrm{Hf}}^{179}$ have been measured by using bremsstrahlung from a 22-Mev betratron as a photon source. The photon-induced reactions were observed by neutron detection supplemented in one case by radioactivity determinations.A comparison of these thresholds with neutron binding energies revealed that, for some of the nuclei, better agreement was obtained if formation of the residual nucleus in an excited state instead of in the ground state was assumed. The neutron binding energies inferred from these measurements are: 11.34\ifmmode\pm\else\textpm\fi{}0.10 Mev for ${\mathrm{Fe}}^{56}$; 7.85\ifmmode\pm\else\textpm\fi{}0.13 Mev for ${\mathrm{Fe}}^{57}$; 6.50\ifmmode\pm\else\textpm\fi{}0.16 Mev for ${\mathrm{Ge}}^{73}$; 10.50\ifmmode\pm\else\textpm\fi{}0.08 Mev for ${\mathrm{Rb}}^{85}$; 9.89\ifmmode\pm\else\textpm\fi{}0.05 Mev for ${\mathrm{Rb}}^{87}$; 6.70\ifmmode\pm\else\textpm\fi{}0.09 Mev for ${\mathrm{Hf}}^{177}$; and 6.52\ifmmode\pm\else\textpm\fi{}0.12 for ${\mathrm{Hf}}^{179}$.

SOME PHOTONEUTRON THRESHOLDS

The photoneutron thresholds for a number of elements which have 100% abundance (or nearly so) in one stable isotope have been measured. The following values in Mev. were found:[Formula: see text]

Precision Determination of Photoneutron Thresholds

The photoneutron thresholds for ${\mathrm{C}}^{12}$, ${\mathrm{N}}^{14}$, ${\mathrm{O}}^{16}$, ${\mathrm{Cu}}^{63}$, and ${\mathrm{Ag}}^{109}$ were determined by measurement of the yield of activity as a function of the peak bremsstrahlung energy. Measurements of the photoneutron thresholds of ${\mathrm{C}}^{12}$, ${\mathrm{N}}^{14}$, and ${\mathrm{O}}^{16}$, which could be calculated accurately from the mass data, served to determine the energy scale. The photoneutron thresholds for ${\mathrm{Cu}}^{63}$ and ${\mathrm{Ag}}^{109}$ were found to be 10.61\ifmmode\pm\else\textpm\fi{}0.05 Mev and 9.07\ifmmode\pm\else\textpm\fi{}0.07 Mev, respectively.The shape of the activation curves for ${\mathrm{C}}^{12}$, ${\mathrm{N}}^{14}$, and ${\mathrm{O}}^{16}$ appeared to be a straight line for about 350 kev near threshold. The activation curves for ${\mathrm{Cu}}^{63}$ and ${\mathrm{Ag}}^{109}$ could be fitted accurately by functions of the form $Y=k{(E\ensuremath{-}{E}_{t})}^{2.23}$ and $Y=k{(E\ensuremath{-}{E}_{t})}^{1.90}$, respectively, over a 3-Mev interval. $E$ refers to the peak x-ray energy and ${E}_{t}$ to the threshold energy.

Photo-Neutron Thresholds of Beryllium and Deuterium

The photo-neutron thresholds of beryllium and deuterium have been measured by comparison with the ${\mathrm{Li}}^{7}(p,n)$ threshold. For this purpose an electrostatic generator was used in which an electron beam and a positive ion beam were established simultaneously. Accurate control and measurement of the generator voltage was obtained by using an electrostatic analyzer on the ${\mathrm{H}}_{2}^{+}$ component of the ion beam. Because the ${\mathrm{Li}}^{7}(p,n)$ threshold lies between the ${\mathrm{Be}}^{9}(\ensuremath{\gamma},n)$ and $\mathrm{D}(\ensuremath{\gamma},n)$ thresholds, errors in extrapolation should be small. The electron beam was accelerated from ground to the accurately known potential of the high voltage electrode where x-rays were produced for the photo-disintegration process. From the photo-neutron thresholds the neutron binding energies of beryllium and deuterium were found to be 1.666\ifmmode\pm\else\textpm\fi{}0.002 and 2.226\ifmmode\pm\else\textpm\fi{}0.003 Mev, respectively.

Experimental Photo-Fission Thresholds inU23592,U23892,U23392,Pu23994, andTh23290

An investigation of the photo-fission thresholds in $_{92}\mathrm{U}^{235}$, $_{92}\mathrm{U}^{238}$, $_{92}\mathrm{U}^{233}$, $_{94}\mathrm{Pu}^{239}$, and $_{90}\mathrm{Th}^{232}$ with a 20-Mev betatron is reported. A method of alpha-self-cancellation is described which permits the detection of fissions and the determination of threshold energies in the two highly alpha-active samples of $_{92}\mathrm{U}^{233}$ and $_{94}\mathrm{Pu}^{239}$. An investigation of the photo-neutron threshold in nitrogen and the energy calculation from mass data provided a check of the absolute energy scale obtained from voltage measurements on the betatron.