Exercise 4: Energy deposition, etc.

Object: Extract energy and flux of transmitted particles from deposition in an object.

This is similar to a combination of Energy Loss and Absorption exercises, but the emphasis is on energy deposited and a new element is added, a 12 thick block of iron. Again, do this for 50 MeV particles.

Remember that the position parameters of boxes in GEANT correspond to the center of the boxes.  The iron block should be positioned such that it is totally contained in the existing VOL1 without overlapping with the target.  Be sure to leave a gap between the target and the block.  If visualization becomes difficult, differentiation of volumes by color can help.

Run for 50 MeV gammas, electrons, pions, and protons. Extract the energy deposition spectrum as well as the mean energy deposition and the number of particles that left energy in the block. The requisite spectrum has already been set up for this exercise, but you might want to review how it was done. Use the PAW HI/FIT command to fit a Gaussian (and if you are feeling adventurous, define a bi-Gaussian function as per HELP HI/FIT and fit with that), or use the PAW LOCATE command, to obtain the energy centroid or peak.

Gammas and electrons will produce a significant shower, whereas the nuclei have a short range and do little more than deposit their energy directly. So see what happens to the energy deposition and number of detected particle when you change the block to a 5cm diameter cylinder (12cm thick, still). Run this for electrons, gammas, pions, and protons, at 50 MeV.

Look for differences in results between the block and the cylinder.  The main effect of interest lies with the electrons and gammas.  Because their energy deposition is dependent on the showering effect, smaller volumes will allow more energy to leak out.  This reduces the resolution of the energy spectrum, or widens the peak.  This effect is the primary limiting factor in gamma ray detectors.  Balancing the necessary large volume against the cost of manufacture has always been a problem.

Another significant effect is to reduce the overall "acceptance" of the iron.  Due to a wide spread of particles exitting the target, not as many hit the downstream face of the iron, resulting in underestimated count rates.  Monte Carlo similations are used just like this to correct the measured count rate of real experiments.

Now generate a 2D histogram (HBOOK2) of the energy deposition in the target vs. the block for gammas, protons, deuterons, and tritons at 30, 40, and 50 MeV. If you overlay all 2D spectra, you should see evidence for bands which result from different energy loss rates for different particles.

This property is often used to discriminate different reaction products, to isolate the reaction of interest.




  [Home] [Next] [Hints & Tips]