Ulysses HISCALE Pages
Document source: Juan Gomez, University of Kansas
We are interested in the g-ray response of silicon detectors on Ulysses. Specifically, we want to produce a reasonable energy loss spectrum from RTG.
We compare the simulated spectra with the measured spectra from some selected "quiet" times, and determine if the background is a result of RTG.
Although there are several methods used to simulate detector performance , we will use a Monte Carlo type algorithm, which is well suited to simulate the natural stochastic processes in gamma ray interactions .
When penetrating a silicon detector, g-rays can interact with the atoms in various ways. The two processes we must consider are:
1.) Compton scattering
2.) pair production
Using the interaction cross sections, sc and sp-p, one can obtain the probability that each interaction will occur . The type of interaction a photon undergoes is then randomly determined, consistent with the relative probabilities of the interactions at the specified energy.
Once the type of interaction is determined, we must then generate a random interaction path length, based on the mean free path of the photon for the interaction and the interaction cross section. We then check the position of the photon to see if the interaction length will take it out of the detector. This process is then repeated until the photon leaves the detector, or until pair production has occurred. For the dimensions of the silicon detectors used in this study, a photon will undergo at most 2-3 interactions before leaving the detector. The method for tracking the photon inside the detector will be explained in more detail in the next section.
For each photon that interacts with the detector, we add up the total energy deposited inside the detector by the secondary electrons . This quantity is then "binned" in order to simulate the detector response.
4.11.3 The Simulation
We run the simulation for two cases:
1. incidence direction of RTG photons along the axis of the detector
2. incidence direction of RTG photons at 90° to the axis
We use the following RTG spectrum :
|# of Photons||Energy (MeV)|
Next, we determine how the photon behaves when it reaches the detector. This involves a multi-step process:
(i) calculate the Compton angle qc
via the Klein-Nishina formula, and calculate the secondary electron energy .
(ii) update the unit vector of the secondary photon (it now becomes the new "incident" photon) and go to step 3.
(i.) calculate the energy of the secondary
(ii.) assume that no reactions will occur after pair production, and go to step 1.
The results for the LEMS and LEFS detectors are shown in Figures 4.79 through 4.82.
1.) Normalization has not been done.
2.) Shape from RTG axial incidence g's is similar to shape observed in quiet times.
3.) RTG radiation could account for observations if the source intensity is adjusted. A single adjustment could be made for each telescope for distance and another for orientation.
 R. Bell, Nucl. Inst. and Meth., 93, p. A-2 (1971)
 M.Kalos and P. Whitlock, Monte Carlo Methods, pp 129-144, Wiley (1986)
 A. Beiser, Concepts of Modern Physics, Ch.12, McGraw-Hill (1981)
 K. Krane, Introductory Nuclear Physics, Ch. 7, Wiley (1988)
 R.W. Campbell, ISPM RTG Simulation (1980)
Next: 4.12 Calculated Responses of HISCALE Sensors to Omnidirectionally Penetrating Galactic Cosmic Rays
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Updated 6/4/09, T. Hunt-Ward