When the 180 MBq 210Po source was placed before the sensor, the count increased to 56 times background at a separation of 25 mm, 88 times at a separation of 60 mm, and 84 times at a separation of 120 mm, see Table 7-2. When a black-out cloth was placed over the collimator to attenuate radioluminescence, the signal reduced from 56 to 1.5 times the background signal. In line with earlier research [33, 74] this confirms the collimated UVTRON was detecting UVC from the 210Po source. It has already been shown through earlier experimentation that the UVTRON can detect UVC from 210Po
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[33, 74]. However, this is the first time that collimated UVC, reflected onto the UVTRON using a UVC mirror has been detected. It confirms that the collimator design will admit and reflect UVC onto the UVTRON sensor.
By taking measurements with an uncollimated UVTRON at the same distances, the effect of the collimator was investigated. The collimator reduced the signal by 94 % at 60 mm, and 78 % at 120 mm, see Table 7-2. The signals were still far greater than background, 88 and 84 times background respectively. The untypical signal drop off, discussed in the following paragraph, may account for the large difference in collimator effect between the 60 mm and 120 mm results.
Table 7-2: Comparison between the signal between the collimated and uncollimated UVTRON when exposed to 210Po, and the recorded background
60 mm separation
120 mm separation
Collimated - CPS 0.125 0.118
Collimated - Times background 88 84
Uncollimated - CPS 2.013 0.543
Uncollimated – times background 1340 369
𝐂𝐨𝐥𝐥𝐢𝐦𝐚𝐭𝐞𝐝
𝐮𝐧𝐜𝐨𝐥𝐥𝐢𝐦𝐚𝐭𝐞𝐝 ⁄
0.06 0.22
Using a UVC bulb to represent the source, the effect of the collimator was further investigated. The body of the collimator was removed to provide the uncollimated result for comparison (see Figures 7-1 a & b). At 15 cm separation, the uncollimated UVTRON saturated (38 CPS) at all angles (up to 90° was tested) and so the UVTRON itself was shielded, with the mirror reflecting UVC onto the UVTRON sensor. It was found that at 15 cm separation and a 0° angle, the UVTRON saturated both with and without the collimator cylinder in place (see Figure 7-1). However, the drop off in signal was much greater when the collimator was turned at an angle to the source with the
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body in place (collimated) than without (uncollimated) (see Figure 7-4). At 15 cm separation the collimated signal reduced by half over each 5 º s turned between 5° and 20°. The uncollimated signal remained saturated until 30° was reached, and then reduced by just under a half over the next 10°. With the light source at 2 m the collimator had a greater effect on the drop in signal caused by angle, with a drop to 72 % in the first 5°, but a drop to less than 3% of the 0° count at 10°. Without the collimator, the signal dropped by only 10% in the first 20°. It can therefore be seen that the reduction in signal produced by the collimator during the experiments with the radioactive sources is due to the reduced field of view caused by the entrance hole diameter. There is also a reduction caused by the reflection of the signal in the UVC mirror, which reflects 85% of UVC, as was demonstrated by the saturation at all angles when the UVTRON was directly exposed to the UVC bulb at 15 cm. The length of travel inside the collimator is also likely to cause a reduction in signal due to the widening of the collimated light over distance (see Figure 7-2). Modification to the geometry of the collimator and the addition of optics could increase the UVC reaching the UVTRON.
Figure 7-4: Response of UVTRON within collimator and with collimator body removed, at a separation between the bulb and collimator of 2 m and 15 cm.
0 5 10 15 20 25 30 35 0 20 40 60 80 100 A v er ag e co unt s per se co n d
Entry angle relative to detector plane (degrees)
UVTRON response to bulb - with and without signal collimation
Collimated 2 m Uncollimated 2 m Collimated 15 cm Uncollimated 15 cm
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The increase in signal from 25 mm to 60 mm separation, when a reduction would be expected, is likely due to two factors; the collimator being within range of the alpha particles and the geometry of the collimator. An estimate of the average distance of travel of the alpha particles emitted by the 210Po can be calculated using the empirical equation;
𝑅𝑎𝑖𝑟 = (0.005𝐸 + 0.285)𝐸1.5 (7.1) where 𝑅𝑎𝑖𝑟 is the travel of the alpha particle in cm, and 𝐸 is the energy of the alpha particle in MeV. The alpha particles emitted by the 210Po, which are of energy 5.304 MeV, would travel approximately 38 mm. Therefore, some of the particles would impact on the collimator at 25 mm, reducing the path length along which the radioluminescence photons are produced. As the Bragg peak shows, the main energy loss of alpha particles is close to the end of their travel path and so impact on the collimator wall would reduce the total radioluminescence caused by alpha ionisation of the air. The collimator was tested as a ‘black-box’, but the internal geometry extends the travel path of the radioluminescence photons and the mirror attenuates the signal, meaning that the 1 𝑟⁄ 2 drop off in signal which would be expected may be affected by the collimator design.
The uncollimated UVTRON showed a more expected response to distance, with the signal varying less than 10% from the expected signal at 60 mm and 120 mm separation distances. To determine the expected CPS at each distance, equation 7.2 was used to calculate the 1 𝑟⁄ 2 relationship in the drop off in signal;
𝑆2 = 𝑆1( 𝑅1 2
𝑅2 2)
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where S2 is the expected signal, S1 is the recorded signal, R2 is the separation at which S2 is expected, and R1 is the separation at which S1 was recorded. This was applied to the data and the results for both the collimated and uncollimated UVTRONs are shown in Table 7-3. The table shows the actual CPS at each separation, and the expected CPS at different separations. Assuming the actual CPS is the true value and the 1 𝑟⁄ 2
relationship will be observed, the expected CPS is the counts that would be measured at a different separation. For example, at 25 mm the reading was 0.0784 CPS. The expected CPS at 60 mm would therefore be 0.0136 (using equation 7.2). As the actual count at 60 mm was 0.1245 this is 9.14 x the expected count.
Table 7-3: Showing the expected and actual results of exposure to 210Po
If CPS at this separation is true these are the expected CPS at the other separation (difference to expected in brackets)
Collimated Actual CPS Expected at 25
mm Expected at 60 mm Expected at 120 mm 25 mm 0.0784 0.0136 (9.14 x) 0.0034 (34.7 x) 60 mm 0.1245 0.7170 (0.11 x) 0.0311 (3.80 x) 120 mm 0.1182 2.7227 (0.03 x) 0.4727 (0.26 x)
Uncollimated Actual CPS Expected at 95
mm Expected at 60 mm Expected at 120 mm 60 mm 2.0132 0.8041 (0.71 x) 0.5040 (1.08 x) 95 mm 0.5721 1.4843 (1.41 x) 1.2634 (0.42 x) 120 mm 0.5425 0.8656 (0.66 x) 2.1699 (0.93 x)
It can be seen from the table that the actual signal was 93 % of the expected signal at 60 mm and 108 % at 120 mm for the uncollimated UVTRON, meaning the actual and expected values are a close match, whereas for the collimated UVTRON, it was 26 % at 60 mm and 380 % at 120 mm, which shows a marked variation from what
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would be the expected count. This is likely due to the collimator internal design which will affect the distance of travel of the UVC photons.
When the result at 25 mm is considered as S1, the count at 60 mm was 914 % of expected and 3472 % at 120 mm. This is likely due to the close proximity of the source to the collimator, within the alpha travel for the R1 value. As noted earlier, this is likely due to the alpha particles impacting on the collimator before reaching their maximum energy transfer point, and the source being a sphere of radioluminescence with radius equal to the travel of the alpha particles and not a point source.