| 1. |
- Dangtip, Somsak, et al.
(författare)
-
Description of the Medley Code : Monte Carlo Simulation of the Medley Setup
- 1998
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Neutron-induced charged-particle production, i.e., reactions like (n,xp), (n,xd), (n,xt), (n,x3He) and (n,xa), yields a large - and relatively poorly known - contribution to the dose delivered in fast-neutron cancer therapy. At the The Svedberg Laboratory (TSL) in Uppsala, a project is underway to measure these cross sections with a precision required for clinical use. For this purpose, an experimental facility, MEDLEY, is under commissioning. It consists of eight detector telescopes, each being a Si-Si-CsI detector combination. This general design has been selected because it provides reasonable performance over the very wide dynamic range required to detect particles ranging from 5 MeV a particles to 100 MeV protons. A general problem in this kind of experiments is to characterize the response of the detection system. The MEDLEY code has been developed for this purpose. Experimental studies of these kinds of charged-particle reactions show specific features. Some of these need to be optimized by means of, for instance, computer codes, prior to the measurement if good data are to be achieved. Basically, charged particles loose energy along their paths by interactions with the electrons of the material. Particles with low energy or with high specific energy loss are easily absorbed. Systems, which use thick charged-particle production targets to gain desirable count rate, can then detect only charged particles with enough energy to escape the target. Thus, using a thick target results in a degraded energy resolution, and particle losses. Thin targets are required to provide better resolution, but at the cost of low count rates. Registration of the entire energy of the particles reaching the detection system is also an ultimate goal. However, charged particles can interact with detection materials via nuclear reactions, which could result in other species of particles. From the detection point of view, the primary particles are lost and replaced by new types of particles, which may behave differently from their predecessors. It is well known that charged particles traveling in a medium are deflected by many small-angle scatterings. This so-called multiple scattering can be described with a statistical distribution. The fluctuations in energy loss per step, called energy-loss straggling, are modeled in the same way, i.e., assuming a statistical behavior. To get an acceptable neutron beam intensity, a rather thick neutron production target (2-8 mm) is required. This causes an energy spread of the incident neutron beam. In our case, the spread after a 4 mm thick 7Li target for neutron production is of the order of about 2 MeV. To analyze the data and determine the true double-differential cross sections, the above mentioned effects have to be taken into consideration. We have therefore developed a Monte Carlo code, MEDLEY, in FORTRAN language, to simulate the experimental setup taking all relevant physical characteristics into account. In the MEDLEY code, particles, chosen from a given distribution, are followed through the detection system. The particle distribution is obtained by applying a stripping method to the measured spectrum supplied by a user. When the result from the MEDLEY code is in good agreement with the experimental data, the true double-differential cross sections is obtained. If needed, the correction procedure can be iterated. This iteration is performed until the above condition is satisfied. This report presents the features included in the code, and some results. We compare ourresults with those from others where available.
|
|
| 3. |
- Söderberg, Jonas
(författare)
-
Fast Neutron Absorbed Dose Distribution Characteristics int he Energy Range 10–80 MeV
- 1998
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The most widely used non-conventional technique for radiation therapy today is fast neutron therapy. Because the neutrons are uncharged, they do not ionize themselves, but liberate secondary, densely ionizing charged particles like protons and a-particles by nuclear reactions in the tissue. In a typical modern neutron therapy facility, a proton beam of about 70 MeV is incident on a thick beryllium target which gives a penetration of the neutron beam which is similar to that obtained with modern megavoltage x-ray therapy facilities. In neutron dosimetry, the measurements are often difficult to assess due to the LET dependence of the detector response. In a neutron radiation field, the LET spans over 4 decades as seen in figure 1. The corrections necessary can therefore be large. For this reason, Monte Carlo calculations are a good complement to measurements and make it possible to evaluate the measurements to a higher degree of accuracy. Furthermore, in measurements the spatial resolution is determined by the detector size. Using Monte Carlo, however, it is possible to study high dose gradients as well. Due to lack of good cross-section data [1], the optimization possibilities and accuracy in fast neutron therapy are not satisfactory. Therefore, fast neutron therapy cannot compete fairly with other modalities of radiation therapy. In radiation therapy, the aim is to deliver absorbed doses to the tumour within 3.5%. At the same time the neutron kerma factors in carbon have an uncertainty of about 10-15%. Work is now underway in Uppsala, at the The Svedberg Laboratory, to get improved double differential cross-section data for fast neutrons on carbon, nitrogen and oxygen since these are the most common elements in the body. The accuracy in the hydrogen data is already at an acceptable level. In the AAPM report no. 7, TE-liquid is suggested as the reference phantom material while the European protocol uses water. Later, it was suggested that water should be the reference phantom material also for the AAPM protocol and in 1989 the ICRU issued a unified protocol. Since the objective of absorbed dose measurements is the absorbed dose to tissue, the following materials are used in this work; water, TE-liquid, standard soft tissue and adipose tissue. The energies presently used in fast neutron therapy are up to about 70 MeV. So in order to test the suitability of water as a reference material for fast neutrons, the energies used are 10, 15, 20, 40, 60 and 80 MeV. One problem with the neutron therapy beams is that large penumbra effects reduce the physical selectivity of the beam, i.e., it is more difficult, compared to in megavolt photon beams, to concentrate absorbed doses to the tumour volume while sparing healthy tissues. This is mainly due to the thermalization of the neutrons and the secondary gamma radiation released in neutron capture processes. One aim of this work is therefore to explore how the penumbra effects vary with neutron energy, testing the hypotheses that the penumbra decreases as the neutron energy increases. The main aim of this work is to use Monte Carlo techniques to evaluate the penumbra effect due to the transport of secondary particles released by the neutrons in the phantom. Pencil beam data are derived, i.e., the neutron source is assumed to be a point source and the SSD (Source-Skin-Distance) to be infinite. The pencil beam results are integrated to yield results for finite beam areas. Furthermore, the absorbed dose due to photons and their secondary particles will be derived separately and compared to the total absorbed dose as a function of depth. At the reference point, at 5-cm depth, the contribution to kerma rom the different neutron reaction channels will be evaluated by scoring values for the neutron fluence and applying partial cross-section data. Since the codes used are FLUKA and MCNP, a comparison of the results from the codes is made at the energies where both codes are valid, i.e., 10, 15 and 20 MeV.
|
|
