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.
Linköping: Linköping University Electronic Press , 1998. , 16 p.