Gas-to-wall absorbed dose conversion factors for neutron energies of 25 to 250 MeV

Cavity chamber absorbed dose measurements do not usually strictly adhere to the conditions of the Fano theorem and therefore the differences in the gas and wall mass stopping powers must be taken into account. Values of gas-to-wall absorbed dose conversion factors rm,g were calculated for neutron energies of 25 to 250 MeV for detectors with walls of C, O, Mg, Al, Si, Fe, Zr, AlN, Al2O3, SiO2, ZrO2, and A-150 tissue-equivalent (TE) plastic and with gas cavities of acetylene, dry air, Ar, an Ar-CO2 mixture, CO2, isobutane, isobutane-based TE, methane, methane-based TE, propane, and propane-based TE. The rm,g calculations required initial spectral fluences of 1H, 2H, 3H, 3He, and 4He ions released by neutron reactions in the walls, and these were calculated with the Los Alamos High Energy Transport code. Mass-stopping-power data were taken from Ziegler and co-workers. Additional calculations were made in order to test the sensitivity of rm,g to input data from other sources, i.e., ion spectral fluences from the ALICE nuclear reaction code and mass-stopping powers from the recent ICRU evaluation. © 1997 Academic Press

Post-stroke inhibition of induced NADPH oxidase type 4 prevents oxidative stress and neurodegeneration

Ischemic stroke is the second leading cause of death worldwide. Only one moderately effective therapy exists, albeit with contraindications that exclude 90% of the patients. This medical need contrasts with a high failure rate of more than 1,000 pre-clinical drug candidates for stroke therapies. Thus, there is a need for translatable mechanisms of neuroprotection and more rigid thresholds of relevance in pre-clinical stroke models. One such candidate mechanism is oxidative stress. However, antioxidant approaches have failed in clinical trials, and the significant sources of oxidative stress in stroke are unknown. We here identify NADPH oxidase type 4 (NOX4) as a major source of oxidative stress and an effective therapeutic target in acute stroke. Upon ischemia, NOX4 was induced in human and mouse brain. Mice deficient in NOX4 (Nox4(-/-)) of either sex, but not those deficient for NOX1 or NOX2, were largely protected from oxidative stress, blood-brain-barrier leakage, and neuronal apoptosis, after both transient and permanent cerebral ischemia. This effect was independent of age, as elderly mice were equally protected. Restoration of oxidative stress reversed the stroke-protective phenotype in Nox4(-/-) mice. Application of the only validated low-molecular-weight pharmacological NADPH oxidase inhibitor, VAS2870, several hours after ischemia was as protective as deleting NOX4. The extent of neuroprotection was exceptional, resulting in significantly improved long-term neurological functions and reduced mortality. NOX4 therefore represents a major source of oxidative stress and novel class of drug target for stroke therapy

Gas-to-wall absorbed dose conversion factors from 25 to 250 MeV neutrons energy

Gas-to-wall absorbed dose conversion factors r(m,g) were calculated with Bragg-Gray cavity theory for the 25 to 250 MeV neutron energy range. Calculations of r(m,g), were made for propane-based tissue-equivalent gas and walls of C, O, Mg, Al, Si, Fe, Zr, AlN, Al2O3, Zr, ZrO2, and A-150 plastic. Charged particle production in the wall materials was calculated with nuclear model codes. Mass stopping powers were taken from recent tabulations. Above 70 MeV neutron energy the r(m,g) values were found to be nearly constant and to approach the wall-to-gas mass stopping power ratio of minimum ionising particles. Slight energy dependencies were found in r(m,g) below 70 MeV. Uncertainties in r(m,g) are estimated to be less than five per cent below 100 MeV neutron energy and are primarily from the uncertainties of the stopping power data

Experimental kerma coefficients and dose distributions of C, N, O, Mg, Al, Si, Fe, Zr, A-150 plastic, Al2O3, AlN, SiO2 and ZrO2 for neutron energies up to 66 MeV

Low-pressure proportional counters (LPPCS) with walls made from the elements C, Mg, Al, Si, Fe and Zr and from the chemical compounds A-150 plastic, AlN, Al2O3, SiO2 and ZrO2 were used to measure neutron fluence- to-kerma conversion coefficients at energies up to 66 MeV. The LPPCs served to measure the absorbed dose deposited in the gas of a cavity surrounded by the counter walls that could be converted to the absorbed dose to the wall on the basis of the Bragg-Gray cavity theory. Numerically the absorbed doses to the walls were almost equal to the corresponding kerma values of the wall materials. The neutron fluence was determined by various experimental methods based on the reference cross sections of the 1H(n, p) scattering and/or the 238U(n, f) reactions. The measurements were performed in monoenergetic neutron fields of energies of 5 MeV, 8 MeV, 15 MeV and 17 MeV and in polyenergetic neutron beams with prominent peaks of energies of 34 MeV, 44 MeV and 66 MeV. For the measurements in the polyenergetic neutron beams, significant corrections for the contributions of the non-peak energy neutrons were applied. The fluence-to-kerma conversion coefficients of N and O were determined using the difference technique applied with matched pairs of LPPCs made from a chemical compound and a pure element. This paper reports experimental fluence-to-kerma conversion coefficient values of eight elements and four compounds measured for seven neutron energies, and presents a comparison with data from previous measurements and theoretical predictions. The distributions of the absorbed dose as a function of the lineal energy were measured for monoenergetic neutrons or, for polyenergetic neutron fields, deduced by applying iterative unfolding procedures in order to subtract the contributions from non-peak energy neutrons. The dose distributions provide insight into the neutron interaction processes

Kerma measurements in polyenergetic neutron fields

Absorbed dose was measured as a function of neutron energy with a small spherical proportional counter (PC) irradiated in the pulsed-beam broad energy spectrum neutron fields of the Physikalisch-Technische Bundesanstalt (PTB) that were produced with the p+ Be reaction and extended to 20 MeV neutron energy. Time-of-flight (TOF) discrimination methods augmented the traditional microdosimetric pulse height (PH) analysis and yielded absorbed dose as a function of lineal energy y and neutron TOF. Below 0.7 keV.μm-1 lineal energy, unfolding procedures greatly improve the time resolution, e.g. from 225 ns full width at half maximum (FWHM) to 65 ns FWHM at 1.2 keV.μm-1. The overall time resolution from unfolded TOF spectra is approximately 30 ns FWHM. The absorbed dose was normalised to neutron spectral fluence and, on the assumption that kerma is numerically equal to absorbed dose, yielded relative neutron fluence-to-kerma conversion coefficients as a function of energy that are in good agreement with values from previous work

Atomic masses of rare-earth isotopes

A survey is given of decay energies of rare-earth isotopes measured in electron-capture decay by relative P/sub K/ ratios, EC/sub K// beta /sup +/, and EC/ beta /sup +/ ratios. Atomic masses of A=147 isotopes and of /sup 146/Gd and /sup 148/Dy are derived. The masses of these isotopes and of alpha -decaying processors are compared with predictions of current mass formulae. The subshell closure at Z=64 is shown for N=82, and 84 isotones. (33 refs)

Comparison of various dose quantities in tissue and tissue substitutes at neutron energies between 20 MeV and 100 MeV

Using kerma factor data for H, C, N, and O in the neutron energy range from 20 MeV to 100 MeV provided by recent calculations and evaluations, the absorbed dose conversion factor α(t,p) from A-150 tissue substitute plastic (p) to ICRU standard tissue (t) was calculated. The results show a constant value of α(t,p) = 0.92 with uncertainties of about 5% from 20 MeV to 50 MeV and of 7% above 50 MeV. Dose distributions from measurements with low pressure proportional counters are used to determine the mean quality factors in C and O. The dose distributions and the quality factors exhibit strong similarities for C and O. Combined with mean quality factors of A-150 plastic and with evaluated kerma factors, the ratio β(t,p) of dose equivalent in ICRU tissue to that in A-150 plastic was calculated. The result indicates that, due to the excess of C in A-150 plastic, the TEPC may overestimate the ambient dose equivalent by about 15% for neutron energies above 20 MeV. These findings confirm the suitability of the TEPC as dose equivalent meter for radiation protection and also suggest that for neutron therapy dosimetry the excess of C in A-150 plastic should be taken into account by using α(t,p) = 0.92 at neutron energies above 20 MeV