RASTA Results

The light output nonuniformity results from the RASTA apparatus are reasonably consistent with those from the CMT box. Discrepancies arise from the differing detection mechanisms in each apparatus, and from the varying processes by which 0.66 MeV photons (RASTA) and 1 GeV muons interact with the CsI material. These differences are manifested as follows:

• The 0.66 MeV photons from the source are not minimum ionizing particles in the CsI. Their energy is also below the 1.022 MeV ee pair production threshold. Therefore, they interact with the CsI primarily through Compton scattering and the photoelectric effect. Because materials with higher atomic number Z are the most favored for photoelectric absorption in the 1 MeV range, the CsI is particularly responsive to the 0.66 MeV photons in this way.

  Unlike the 0.66 MeV photons, the 1 GeV cosmic muons are minimum ionizing particles, and interact by ionizing the molecules in the CsI. In general, they exhibit very little brehmstrahlung due to their energy and relatively large mass. Any muons which do scatter and emit brehmstrahlung are identified as such, and are ignored to preserve accurate tracking information.

• There is a difference in the distribution of energy deposition between the RASTA and CMT apparatus. The cosmic muons transfer their energy uniformly along their tracks in the CsI material. On the other hand, while the 0.66 MeV photons from the Cs source can probe the entire volume of the crystal, the Monte Carlo simulation shows that they deposit six times as much energy near the crystal surface as they do at the central (z) axis. [8] This feature leads to the extraction of different optical nonuniformity coefficients, because of inherent averaging over varying scintillation volumes in the two methods.
• Finally, the position resolution of each event in the RASTA box is determined by the position of the collimated Cs source, and not through tracking with wire chamber data as in the CMT apparatus.

Figure 3.11 shows the light output collection nonuniformity slopes for hexagonal crystal S034 from both the RASTA and CMT apparatus. There is reasonably good agreement in the region 0<z<10 cm.

  
Figure: Comparison of tomography results for hexagonal crystal s034, using the RASTA apparatus (left panel) and the CMT apparatus (right panel). The three fitted lines in each case depict the nonuniformity results for the regions 0:10 cm, 10:18 cm, and 18:22 cm.

The tomography results for both methods are quantified for crystal S034 in Table 3.1.

  
Table: Comparison of light output collection nonuniformities for crystal S034, measured with CMT and RASTA.