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The photoneutron dose equivalent in a linac radiotherapy room and its entrance maze was investigated by means of Monte Carlo simulations under different conditions. Particularly, the effect of neutron absorbers and moderator layers placed on the maze walls was considered. The contribution of prompt gamma rays emitted in absorption reactions of thermal neutrons was also taken into account. The simulation results are compared with some experimental measurements in the therapy room and in the maze.

Passive and active detection techniques have been employed in order to measure neutron fluence rates and corresponding exposure rates around medical electron accelerators operating at energies well above the neutron binding energies of the structural materials. In these conditions from the treatment head, in the direct photon flux and from the shielded region, a fast neutron flux emerges which is partly absorbed and partly scattered by the walls, eventually establishing a nearly uniform thermal and epithermal flux in the room. Both direct and scattered flux contribute to the dose to the patient. A smaller neutron dose rate can also be found outside the treatment room, where the therapy staff works. Passive detectors, of moderation type, have been employed in the treatment room and {sup 3}He active detectors in the external zones. For the treatment room the activation data were compared with results of Monte Carlo simulation of the neutron transport in the room. Technical features of the two measures are briefly presented and results obtained around three different types of accelerators are reported. At the higher beam energies, i.e., 25 MV, a neutron dose of 0.36 Sv was estimated in the treatment field in addition to a therapeutic x-ray dosemore » of 50 Gy. At lower energies or out of the treatment field the neutron dose drops significantly. In the external zones the dose rates everywhere are below the recommended limits and normally very low, the highest values being recorded in positions very close to the access door of the treatment room.« less

Results obtained in more than 100 electrolysis experiments of D2O with palladium and titanium cathodes, characterized by continuous high-efficiency neutron monitoring, are reported. In two runs wit...

Passive and active detection techniques have been employed in order to measure neutron fluence rates and corresponding exposure rates around medical electron accelerators operating at energies well above the neutron binding energies of the structural materials. In these conditions from the treatment head, in the direct photon flux and from the shielded region, a fast neutron flux emerges which is partly absorbed and partly scattered by the walls, eventually establishing a nearly uniform thermal and epithermal flux in the room. Both direct and scattered flux contribute to the dose to the patient. A smaller neutron dose rate can also be found outside the treatment room, where the therapy staff works. Passive detectors, of moderation type, have been employed in the treatment room and 3He active detectors in the external zones. For the treatment room the activation data were compared with results of Monte Carlo simulation of the neutron transport in the room. Technical features of the two measures are briefly presented and results obtained around three different types of accelerators are reported. At the higher beam energies, i.e., 25 MV, a neutron dose of 0.36 Sv was estimated in the treatment field in addition to a therapeutic x-ray dose of 50 Gy. At lower energies or out of the treatment field the neutron dose drops significantly. In the external zones the dose rates everywhere are below the recommended limits and normally very low, the highest values being recorded in positions very close to the access door of the treatment room.

The propagation of neutron flux and cross-section covariances through a group structure variation is investigated and discussed. Explicitly taking into account the initial covariances between diffe...

The present paper deals with the statistical errors affecting the reactivity estimate in rod-drop and rod-jerk experiments. Within the framework of the one-velocity point-reactor model with delayed neutrons and using the Kolmogorov theory of the branching stochastic processes, expressions for the mean values and the covariance matrix of the counts taken in such measurements were obtained. Various experiments were simulated and then analysed with a simple classical model here revisited. In all cases the estimated reactivities were in very good agreement with the ‘true’ ones. The importance of using the correct covariance matrix was then checked by treating the pseudo-experimental counts as if they were Poissonian. In the case considered here it turned out that this simple assumption, while yielding acceptable point estimates for the reactivities, delivers unreliable values for their standard deviations, thus possibly inducing the assessment of non-existent discrepancies.

Cross-section libraries and input neutron spectrum data are generally required in the application of activation neutron spectrometry and are usually given in the form of group values. Fine structures (620 groups for the energy range from 10/sup -10/ to 18 MeV) are often used. but problems arise if adjustment codes implying matrix calculations are adopted. In this Note, the errors deriving from the use of a coarse (40-group) structure are estimated and discussed making special reference to a fast neutron spectrum.

The divergence of the variance and covariance of the neutron counts in a critical reactor has been investigated in the usual one-speed point-reactor model, explicitly taking into account the delayed neutrons. The statistical approach is based upon the Kolmogorov and Dmitriev theory of the branching stochastic processes, making use of the probability-generating-function formalism. The analysis is performed by considering an ensemble of critical reactors steady at the same power level—an effect of the control system having been in operation for a long time. At the start of the experiment the reactors are left free to drift in the absence of any feedback mechanism. The influence of the assumptions of the initial distributions of the neutron and precursor populations is investigated and found to be negligible. After an initial transient lasting for a time of the order of the inverse of the Rossi-α, both the variance and covariance linearly increase with time with a common slope. From a mathematical point of view, in the limit of time going to infinity, they obviously diverge, giving rise to the ‘critical catastrophe’. However, in actual experiments lasting a few tens of seconds, the relative increase is confirmed to be negligible.

The results of a measurement of periods and yields of delayed neutrons from “fast” fission of 237Np, 238Pu, 241Am, 233U, 240Pu, and 241Pu are presented. For the first three nuclides, only a few inc...

γ and X radiation accompanying the decay of82Sr have been investigated. γ rays of 775 and 1 400 keV have been found;82Rb daughter of82Sr, is shown to decay through β+ emission, not only to the ground state but also to excited levels of82Kr.

Two unfolding methods, SAND II and SPECTRA, are tested and compared. The experimental results obtained with nine threshold detectors irradiated in the centre of the core of the L54 reactor are used as input data. The relative merits of the two methods are presented and discussed.

Summary The possibility of different behaviours of 60Co and 60mCo in the Szilard-Chalmers recoil was investigated and proved in [Co(NH)3)6] (NO3)3 and trans - [Co en2(NO2)2] NO3. It was found that the retentions of 60Co and 60mCo atoms in the original compounds, for irradiation time approaching zero, are different. Particularly the ratio of i0B,Co to eoCo retentions at zero irradiation time is 0.90 + 0.02 for [Co(NH3)5] . (NO3)3 and 1.26 + 0.04 for trans - [Co en2(NO2)] NO3. The annealing kinetics of recoiled 60Co and 60mCo atoms under fast neutron and γ-ray irradiations at a temperature of 40 °C are also different.

A neutron spectrum adjustment with the multiple foil activation technique has been performed on two fast neutron facilities.

The results of discrete ordinate DOT calculations are compared with Multiple Foil Activation measurements performed at RB-2/TV research facility (Montecuccolino Laboratories, Bologna) and at Caorso power plant (Caorso, Piacenza).

Foil intercalibration factors in neutron flux measurements are determined, together with relative flux values, as a result of different irradiations with rotation of the irradiation positions of the different foils.