Size (and shapes) does matter!?
One of the outstanding issues is hwy to choose what type of detector. Over the years we have done a lot of studies looking at the optimum size and shape of a detector. Of course this depends on the type of application, the type of crystal and the size and weight constraints. In one of the studies we were interested in the optimum diameter of a detector. In other words, why use 4x4 inch square detectors instead of round ones? Or the other way around, does it improve the overall efficiency using circular detectors instead of square ones?
In order to find out we did MC sims of s series of detector types and sizes inside our calibration setup. And we compared the results, as shown below.
The test
In the test we compare the 232Th spectrum resulting from a 1 Bq/kg source of 1m3 volume around the sensor. All calculated using MCNP models. We then checked the counts a detector gets per second from our simulated 232Th source. To compare the efficiency, we normalized the volume of detector material to the volume of the "standard" 4L CsI MS-4000 system. Table 1 lists the results.
Table 1. Detector types tested and results.
| 5x6" BGO | 3x8.6" CsI | 3.5x6.5" CsI | 3.5x13" CsI | 4x4x16" CsI | |
| Volume (ml) | 1931 | 996 | 1025 | 2050 | 4198 |
| Density (kg/l) | 7.13 | 4.51 | 4.51 | 4.51 | 4.51 |
| Shape | round | round | round | round | square |
| Counts per Bq/kg-1 | 11.60 | 6.59 | 6.38 | 12.55 | 19.82 |
| Counts per Bq/kg-1 per kg detector | 0.84 | 1.47 | 1.38 | 1.36 | 1.05 |
| % in 2360 keV peak | 9.37% | 4.55% | 4.93% | 5.35% | 6.05% |
| Relative efficiency w.r.t. 4x4x16 | 128% | 140% | 131% | 129% | 100% |
What do we learn from this?
- The relative efficiency of all circular detector is better!
- The peak-to-Compton ratio of the BGO is the highest. This makes sense, the BGO has much higher Z and therefore a higher photopeak efficiency. However, the thinner 3.5 inch crystals still have a very acceptable photopeak efficiency of > 5%.
- The highest count rate gain is seen with the 3x8.6 inch CsI detector. This shape seems optimal for count rate. However, its peak efficiency is lowest.
- The overall best systems are the 3.5 inch diameter systems.
In conclusion we see that moving from a single 4x4x16 system to a double 3.5x13 inch system increases your gain by 30% without loosing too much peak efficiency! In other words, 30% more value for money!