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Treatments delivered by proton therapy are affected by uncertainties on the range of the beam within the patient, requiring medical physicists to add safety margins on the penetration depth of the beam. To reduce these margins and deliver safer treatments, different projects are currently investigating real-time range control by imaging prompt gammas emitted along the proton tracks in the patient. This study reports on the feasibility, development and test of a new concept of prompt gamma camera using a slit collimator to obtain a one-dimensional projection of the beam path on a scintillation detector. This concept was optimized, using the Monte Carlo code MCNPX version 2.5.0, to select high energy photons correlated with the beam range and detect them with both high statistics and sufficient spatial resolution. To validate the Monte Carlo model, spectrometry measurements of secondary particles emitted by a PMMA target during proton irradiation at 160 MeV were realized. An excellent agreement with the simulations was observed when using subtraction methods to isolate the gammas in direct incidence. A first prototype slit camera using the HiCam gamma detector was consequently prepared and tested successfully at 100 and 160 MeV beam energies. Results confirmed the potential of this concept for real-time range monitoring with millimetre accuracy in pencil beam scanning mode for typical clinical conditions. If we neglect electronic dead times and rejection of detected events, the current solution with its collimator at 15 cm from the beam axis can achieve a 1-2 mm standard deviation on range estimation in a homogeneous PMMA target for numbers of protons that correspond to doses in water at the Bragg peak as low as 15 cGy at 100 MeV and 25 cGy at 160 MeV assuming pencil beams with a Gaussian profile of 5 mm sigma at target entrance.

Many experiments have shown that the backward yield of secondary electrons induced by protons in solid targets is proportional to the electronic stopping power Se: γB = ΛBSe. The parameter λb can be considered as a “material parameter” because it is constant over a wide proton energy range. The forward yield, which can be measured with very thin targets, seems to exhibit the same property. Using a Monte Carlo model for the electron transport in aluminium, we have shown that the anisotropy of the angular distribution of the electrons excited by the proton plays a dominant role together with the number of elastic collisions suffered by an electron before it escapes. When this number is low (forward emission at low proton energy), the yield is approximately proportional to the total excitation cross section and, when it is large (backward emission), the yield is proportional to the stopping power.

Experiments on the secondary emission of electrons from thin carbon foils bombarded with protons have shown that the forward-backward yield ratio increases with energy below 200 keV. Theoretical calculations for an aluminium target, on the contrary, show that this ratio decreases with energy. To explain this discrepancy, we have included in our Monte Carlo simulation model the electron capture and loss process, assuming that a proton-captured electron system does not excite electrons in the solid. We have also taken into account the partial yield due to the lost electrons. This simple model, applied to aluminium, gives results consistent with the carbon experiments. We have also studied the statistics of the number of secondary electrons accompanying an outgoing proton or hydrogen atom.

Ion induced secondary electron emission is often studied by assuming that the internal electron source is spatially uniform and that the medium is infinite. With this simplification, the Boltzmann equation is simply an integral equation, the only variable being the electron energy. Comparisons between this model and Monte Carlo calculations for a semi-infinite medium show that the secondary electron yield is overestimated by about 25%.This effect is due to the surface. We propose an approximate method to calculate the surface correction to the infinite medium electron current. This method starts from the integral form of the transport equation, which takes the partial reflection boundary condition into account, and it uses the infinite medium electron flux. The final expression gives the electron current correction in terms of an integral over the energy variable. We use this expression to estimate the surface correction to the electron yield for protons on aluminum.

Gamma rays and conversion electrons following thermal-neutron capture in 75 As have been measured using the Ge pair spectrometer PN4, the bent crystal spectrometers GAMS and the β -spectrometer BILL at the ILL. Coincident γ -rays were recorded at the CEN/SCK Mol, and average resonance capture at E n = 2 and 24 keV has been studied at the HFBR of BNL. The neutron binding energy in 76 As was determined to be 7328.498 ± 0.075 keV. The resulting level scheme for 76 As greatly improves the knowledge on this odd-odd nucleus. The low-lying positive-parity states are discussed in the U(5) limit of the U v (6/12)⊗U π (6/12) supersymmetry using the same hamiltonian to describe the quartet 76 Se- 77 Se- 75 As- 76 As having the same total number of bosons plus fermions.

We investigate the validity of the surface electric-field interaction model in beam-foil experiments where the polarization of the emitted light is measured. After summarizing the theory for expanding the density operator into state multipoles and linking these to the Stokes parameters of the light, we show also how to use symmetries to reduce the size of the calculations. A very simple model is chosen for the electric field: it is perpendicular to the foil and constant over a finite distance, then zero. This model is used to find the initial density matrix for the {ital n}=3 singlet states of He I and the electric-field parameters from tilted-foil experiments. Reasonable results are obtained given the unrealistic electric-field model used.

The authors have measured the Stokes' parameters for the foil-excited 3p 1P and 4d 1D levels of He I as a function of foil tilt angle and beam energy. The previously observed tilt angle invariance of one of the alignment parameters has also been observed here. The results have been compared with the surface electric field model.

We have measured the linear polarization of the Hβ transition at 486.1 nm excited by passage of a 110 keV proton beam through perpendicular carbon foils. We have observed that the polarization depends upon the beam intensity and on the relative position of the foil and its holder. We have shown that these dependences are linked to the presence of a stray electric field at the immediate vicinity of the foil. The field is due to the potential distribution at the foil surface resulting from the electron radial flow in the high foil electric resistance (about 50 kΩ). It introduces a perturbation which in our case is more important than the temperature effect observed by Gay and Berry (Phys. Rev. A19, 952 (1979)). The field is proportional to the beam current density and is reduced for large foil and beam diameters.

We have calculated the coherent excitation cross sections induced in electron capture collisions with fast impinging ions, within the eikonal approximation taken in its prior form. The cross sections and components ϱ0k(l1, l2) of the irreducible tensor associated with the l1, l2 density matrix elements, are given in a closed analytical form, which yields directly the total intensity and the polarization. This result generalizes the one obtained by F.T. Chan and J. Eichler (Phys. Rev. A20 (1979) 1811) for electron capture into arbitrary nl shells. The sub-shell cross section values agree with those calculated within the Born approximation. The polarization — about 20% — is higher than that obtained in close coupling methods (i.e. H.G. Morrison and U. Opik, J. Phys. B11 (1978) 473). It is also higher than that measured in electron capture experiments with thin gas-cells.

The foil tilt angle dependence of the Stokes parameters has been calculated assuming the existence of a strong electric field perpendicular to the foil surface. The second order results obtained by Y.B. Band (Phys. Rev. 13 (1976) 2061) have been generalized by taking into account the energy separations between the levels. This has lead to a supplementary tilt angle dependent term which is linear in the electric field and the energy differences. The most important consequence of this term is that the Stokes parameters C and S are no longer proportional which is in agreement with the experimental results for the 4d 1D level in He I (R.M. Schectman et al., Phys. Rev. 22 (1980) 1591). The results of this second order perturbation calculation have been compared with the Stokes parameters evaluated exactly. This was done by resolving numerically the set of differential equations for the density matrix taking into account the coupling between four levels (s, p, d, f).

The secondary electron yield and the mean charge of the beam have been measured as a function of the beam current. We have shown that the observed current dependence is not a temperature effect.

Direct and coincidence gamma spectroscopy has been applied to the reaction187Os(n, γ)188Os. A neutron binding energy of 7,989.3±0.3 keV is deduced and a level scheme is established. The experimental level energies andB(E2) ratios are compared to calculations in the framework of the Interacting Boson Model.

Theoretical predictions of three electron transport models are compared for H+ induced secondary electron emission from Al. In particular, the secondary electron energy distribution and the ion energy dependence of the secondary electron yield have been obtained. For thin targets, we have calculated the forward and backward electron yields as well as their dependence upon the angle of incidence of the ions. Our results are in agreement with the presently available experimental data. However, further work is needed to describe adequately the thin foil results.

We have observed zero-field quantum beats, for the H β transition, in gas-excited hydrogen using a short gas cell of He and Ar. The measurements have shown evidence for an s-d coherence amplitude as important as the one obtained with a carbon foil target.

The improved age-diffusion model for secondary-electron transport is applied to aluminum. Electron cross sections for inelastic collisions with the free-electron gas using the Lindhard dielectric function and for elastic collisions with the randomly distributed ionic cores are used in the calculations. The most important characteristics of backward secondary-electron emission induced by low-energy electrons on polycrystalline Al targets are calculated and compared to experimental results and to Monte Carlo calculations. The model appears to predict the electronic yield, the energy spectra, and the spatial dependence of secondary emission with reasonable accuracy. © 1987 The American Physical Society.

When fast hydrogen atoms emerge from a thin carbon target, they interact with an electric field produced by the target itself. We have studied this interaction by measuring the total intensity and polarization of the Balmer lines as a function of the beam current. We have shown that the electric field increases with the beam current and acts over a distance of a few millimeters beyond the foil. A theoretical model has been developed. It reproduces the main features of the experimental data.

We have developed in this paper a semianalytical electron transport model designed for parametric studies of secondary-electron emission induced by low-energy electrons (keV range) and by fast light ions (100 keV range). The primary-particle transport is assumed to be known and to give rise to an internal electron source. The importance of the nearly isotropic elastic scattering in the secondary-electron energy range (50 eV) and the slowing-down process strongly reduce the influence of the anisotropy of the internal electron source, and the internal electron flux is nearly isotropic as is evidenced by the experimental results. The differential energy behavior of the inelastic scattering kernel is very complicated and the real kernel is replaced by a synthetic scattering kernel of which parameters are obtained by energy and angle moments conservation. Through a P1 approximation and the use of the synthetic scattering kernel, the Boltzmann equation is approximated by a diffusion slowing-down equation for the isotropic part of the internal electron flux. The energy-dependent partial reflection boundary condition reduces to a Neumann-Dirichlet boundary condition. An analytical expression for the Greens function of the diffusion slowing-down equation with the surface boundary condition is obtained by means of approximations close to the age-diffusion theory and the model allows for transient conditions. Independently from the improved age-diffusion model, a correction formula is developed in order to take into account the backscattering of primary electrons for an incident-electron problem. © 1987 The American Physical Society.

A model has been developed which gives explicit formulation of the energy, radial, time and angular dependence of outgoing secondary electrons, some of which were previously available only through Monte Carlo simulation. The cross-sections are arbitrary as well as the source. The model has been tested by evaluating some static characteristics and comparing them with other available experimental and theoretical results. The general agreement is fairly good. Numerical results for time dependence will be investigated next.

The natural linewidth of the 24 keV level of 119 Sn m has been remeasured by Mossbauer spectroscopy and delayed coincidence giving a half life of 23 ns greater than the standard value of 18.4 ns.