摘要:

Measurements of a 670‐MeV/amu20Ne beam at the Lawrence Berkeley Laboratory Bevalac heavy‐ion accelerator with various thicknesses of water absorber were obtained with the BERKLET. The BERKLET, a simple three‐stage solid‐state telescope detector, has been described previously. This instrument measures the linear energy transfer (LET) and residual energy of particles, allows the identification of the particle's charge, and provides a means of obtaining LET and energy statistics for the beam, separated by particle charge. The track and dose averaged LET dependence on the amount of water absorber was determined for each species of fragment in the beam. Large numbers of low‐LET particles in the fragmented beam were detected. The results of the analysis are presented followed by a discussion of the effects of multiple scattering and secondary fragmentation on the measurements. A brief discussion of the implications of the BERKLET measurements for radiobiology is also presented.

ISSN:0094-2405    

DOI:10.1118/1.596536

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Accelerated heavy‐ion beams used in biological and medical research are often utilized in conjunction with absorbers which lead to the fragmentation of the beam. The BERKLET, initially a two‐stage solid‐state telescope detector, was designed to make rapid, on‐line energy and linear energy transfer (LET) measurements of individual particles in a heavy‐ion beam, thus allowing characterization of fragmented beams. From data collected with the BERKLET, one is able to determine a number of important parameters. These include: residual energy and LET histograms for the full beam and for the individualZcomponents, relative number of particles with a givenZ, and dose and track average LET's for the full beam and for the individualZ's. Improvements to the BERKLET design and changes in data analysis are discussed and contrasted with the results of an earlier BERKLET configuration. The most notable improvements are the addition of a thin scintillation detector for improved LET measurement, a tenfold improvement in the dynamic range of the event discriminator, reported here as 1:2000, and dual high‐and low‐gain amplification of the LET signals, permitting the identification of particles withZ's ranging from 12 down to 1.

ISSN:0094-2405    

DOI:10.1118/1.596537

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An experiment was performed at the Lawrence Berkeley Laboratory BEVALAC to measure the multiple Coulomb scattering of 650‐MeV/A uranium nuclei in 0.19 radiation lengths of a Cu target. Differential distributions in the projected multiple scattering angle were measured in the vertical and horizontal planes using silicon position‐sensitive detectors to determine particle trajectories before and after target scattering. The results were compared with the multiple Coulomb scattering theories of Fermi and Molière, and with a modification of the Fermi theory, using a Monte Carlo simulation. These theories were in excellent agreement with experiment at the 2σ level. The best quantitative agreement is obtained with the Gaussian distribution predicted by the modified Fermi theory.

ISSN:0094-2405    

DOI:10.1118/1.596538

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Measurements of the effect of either gold or silver backing on the dose rate around an125I seed were performed using a Therados RFA7 dosimetry system and a small diode detector which was 2.5 mm in diameter and 0.06 mm thick. It was found that the presence of the gold or silver backing modifies the diode response on the side of the125I seed away from the backing. The effect depends on the backing material and the distance from the seed. There is a small increase close to the gold backing but a decrease further away. This decrease at distances greater than 10 mm from the seed is uniformly 10%, the same as found when the seed is backed by air. There is an increase of up to 25% observed with silver backing the seed and this increase remains significant more than 30 mm from the seed. When the response increases, the results are hard to interpret quantitatively because of variations in the diode response per unit dose with photon energy and extreme sensitivity to geometric changes. Nonetheless, except for the increase at close distances with the gold, the results are in agreement with EGS4 Monte Carlo photon transport simulations which are for a simplified geometry and account for x‐ray fluorescence from theK‐shell. Furthermore, the increase in the gold‐backed case is qualitatively explained by Williamson's Monte Carlo calculations which take into account theL‐shell fluorescent x‐rays from gold.

ISSN:0094-2405    

DOI:10.1118/1.596588

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The EGS4 Monte Carlo code has been used to simulate the emission and energy deposition in H2O about point sources of monoenergetic electrons and radionuclides of potential use in radioimmunotherapy. The radiations studied were 0.05, 0.1, 0.5, 1.0, 2.0, and 3.0 MeV monoenergetic electrons and32P,67Cu,90Y,105Rh,131I,153Sm,186Re, and188Re β particles and conversion and Auger electrons. Ten batches of 10 000 electrons (or 10 000 radionuclide decays) each were started isotropically at a point in an infinite homogeneous H2O phantom. The parameter–reduced electron–step transport algorithm (PRESTA) version of the EGS4 Monte Carlo code was used to follow these and their progeny, scoring the energy deposited in thin spherical shells. The scaled dose kernels are calculated and compared to kernels available in the literature. These previously published kernels either completely ignore secondary electrons or are based on a Monte Carlo code which improperly sampled the Landau energy straggling distribution.

ISSN:0094-2405    

DOI:10.1118/1.596565

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In the convolution/superposition method of photon beam dose calculations, inhomogeneities are usually handled by using some form of scaling involving the relative electron densities of the inhomogeneities. In this paper the accuracy of density scaling as applied to primary electrons generated in photon interactions is examined. Monte Carlo calculations are compared with density scaling calculations for air and cork slab inhomogeneities. For individual primary photon kernels as well as for photon interactions restricted to a thin layer, the results can differ significantly, by up to 50%, between the two calculations. However, for realistic photon beams where interactions occur throughout the whole irradiated volume, the discrepancies are much less severe. The discrepancies for the kernel calculation are attributed to the scattering characteristics of the electrons and the consequent oversimplified modeling used in the density scaling method. A technique called the kernel integration technique is developed to analyze the general effects of air and cork inhomogeneities. It is shown that the discrepancies become significant only under rather extreme conditions, such as immediately beyond the surface after a large air gap. In electron beams all the primary electrons originate from the surface of the phantom and the errors caused by simple density scaling can be much more significant. Various aspects relating to the accuracy of density scaling for air and cork slab inhomogeneities are discussed.

ISSN:0094-2405    

DOI:10.1118/1.596497

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper describes a method of calculating three‐dimensional (3‐D) photon primary absorbed dose in a homogeneous or heterogeneous medium. The method is based on a technique of convolving a pair of forward and backward spread dose‐distribution functions with the primary water collision kerma distribution. Both spread dose‐distribution functions can be constructed by analyzing the zero‐area tissue‐maximum ratio, the primary absorbed dose near the beam exit surface, and the laterally spread dose distribution. Primary absorbed dose calculations are performed along the beam axis for 10‐MV x rays. The characteristic patterns of primary dose in heterogeneous phantoms containing an aluminum or lunglike material slab can be obtained.

ISSN:0094-2405    

DOI:10.1118/1.596498

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Three‐dimensional (3‐D) forward and backward primary dose spread functions in water were developed for 10‐MV x rays. Three‐dimensional forward and backward primary dose spread functions in a heterogeneous medium were constructed from the ones in water using the density scaling theorem. Each of the forward and backward primary dose components were calculated using a method of convolving the primary water collision kerma distribution with the forward or backward primary dose spread function. Scatter dose was separated into forward and backward components. Each scatter dose component was calculated using a differential scatter method, a kind of convolution method, where the primary water collision kerma distribution was convolved with a differential scatter‐maximum ratio or differential backscatter factor equation. From the dose calculation and measurement results obtained for water phantoms containing a cork or aluminum slab, it was found that the 3‐D forward and backward primary dose functions were effective especially in regions where there was a loss of longitudinal and/or lateral electronic equilibrium.

ISSN:0094-2405    

DOI:10.1118/1.596499

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An improved, quantitative version of the partial fluence model [Med. Phys.14, 585 (1987)] for the calculation of dose perturbations at media interfaces in photon beams is presented and compared with measurements made at interfaces between polystyrene and materials ranging in atomic number from aluminum to lead, for photon beams ranging in energy from60Co to 24 MV.

ISSN:0094-2405    

DOI:10.1118/1.596500

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A numerical method for the reconstruction of bremsstrahlung spectra has been applied to the analysis of simulated data. The method is found to be reliable in reconstructing x‐ray spectra of maximum energy up to 10 MeV. Measurements of aluminum transmission data carried out for five linear accelerators have also been analyzed by this method to determine the fractional energy fluence. The values of the Spencer–Attix water/air stopping power ratio,S w,airS.A., reported as a function of the ratiosTPR1020, are in good agreement with computed results. Quality index values were correlated to mean incident energies of x‐ray beams used in radiotherapy. The experimental setup has also provided information regarding the softening effect on the off‐axis beam, which can be used in clinical computer dosimetry to correct conventional zero field size tissue maximum ratio.

ISSN:0094-2405    

DOI:10.1118/1.596567

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY