With this simulation program the optimization of the radiation shielding  of a measuring laboratory can be performed, which is close to a research reactor. The optimal configuration of the shielding materials should be found, where the dose rate in the measuring room is minimal.

 The source of the radiation is a small research reactor (left side of the figure). In a so-called "hot chamber" (measuring room), which is located close to the reactor, measurements of freshly irradiated samples can be made. In order to protect the health of the personnel it is important that the dose rate in this room caused by the reactor should be as small as possible. For simplicity, the effect of the natural background radiation is neglected in the simulation. Since we examine only the effect of shielding materials, the decrease of the radiation fields due to the distance is neglected as well.

The types of the radiation
Three types of radiation are distinguished from the point of view of radiation protection:
1) fast neutrons,
2) thermal (slow) neutrons and
3) gamma photons.
(The beta and alpha particles are absorbed very quickly in the reactor and the surrounding materials, so their effect can be neglected.) The health effect of each radiation is different, therefore we use different weight factors when calculating their contribution to the equivalent dose. The weight factor for gamma-photons is one, that of the thermal neutrons (in this program) is 5, while the weight factor of the fast neutrons is 20. The total dose can be calculated as a weighted sum of the contributions of the different radiation fluxes:
D ~ flux_gamma + 5*flux_thermal + 20*flux_fast.

In the simulation the starting value of each radiation flux can be selected using three vertical sliders between 50 and 200 (arbitrary units).The slider denoted by F changes the initial flux of the fast neutrons, the one denoted by S changes the flux of the slow neutrons, whereas the slider denoted by G changes the initial flux of the gammas.

Important processes for the radiation shielding 
Different processes occur in the materials used for shielding. Each of the three types of radiation can be absorbed (after this process nothing comes out), the fast neutrons can be slowed down (this increases the flux of the slow neutrons) or they can be captured inducing gamma emission. The thermal neutrons can also induce gammas, when they are captured. The probability of these 6 different processes in unit thickness is different in every shielding material. The 3x6 = 18 probability parameters built in the simulation are based on realistic measurements and on professional simulations.

Shielding materials 
Three types of materials are available:
1) paraffin,
2) boric acid (dissolved in water), and
3) lead.
These three substances are widely used in radiation protection, however they have very different behaviour concerning the processes described above. The paraffin is an excellent moderator (slows down the neutrons), the boric acid is a moderator but also a neutron- and gamma-absorbent, and the lead is a very efficient gamma-absorbent. The simulation gives some help to the user by plotting the variation of the three radiation fluxes along the different shielding materials.

Goal
Our task is to find the correct sequence and appropriate thicknesses of the layers, which can be placed  in the 100-centimeter space available, which minimize the dose rate. We get some help from the available measuring devices. We have a dose-rate meter, a particle counter, and a dosimeter that measures total total dose (the integral of the dose-rate during the measuring time). Please note that the detection efficiency of the three types of radiation in the particle counter is not the same as the radiation factors for the health effects! Since the nuclear processes have stochastic character, the counting rate of every detector fluctuates around their expectation values. In order to decrease the scatter of the measurements (and thus the uncertainty of the optimum value), a minimal measuring time is needed.  The determination of this measuring time is also part of the task. Try to find a measuring time where the scatter of the results will be lower than 10%. The average dose-rate from the natural radiation background is about 100 nSv/h. The radiation shielding is acceptable if the dose rate coming from the reactor is smaller than this.      

The simulation is a slightly extended version of the simulation excercise of the Leo Szilard Physics Competition in 2016. (http://www.szilardverseny.hu/orszagos-verseny/feladatsorok)