PAT Radiation Question
This is a question from the PAT specimen paper from 2009:
A radioactive source is placed 6 cm from a radiation detector sensitive to all forms of ionising radiation, and records 74 counts/minute. When a 1 cm thick aluminium plate is placed in the gap then the count rate falls to 45 counts/minute. If the source is removed entirely then the count rate remains the same. If the aluminium plate is removed and the source is placed 2 cm from the detector, the count rate rises to 5000 counts/minute. What forms of radiation does the source emit?
A) Alpha only
B) Beta only
C) Alpha and beta
D) Alpha and gamma
The PMT solutions for this paper claim it is alpha only, but I do not see how this can be the case. Surely it is alpha and beta?
The count when there is a 1 cm aluminium plate present (which should block alpha and beta, but not gamma) is the same as when there is no source (therefore the background radiation) - therefore, there is no gamma radiation coming from the source.
When the detector is 6 cm from the source, the count is at 74 cpm. This is 29 cpm higher than the background. At this distance, beta radiation should reach the detector, but not alpha. This suggests the source is emitting beta radiation.
Then when the detector is really close to the source, the count is much higher, suggesting there is also alpha radiation, that could not previously reach the detector.
How, therefore, is it only alpha radiation? Surely there is beta there as well?
A radioactive source is placed 6 cm from a radiation detector sensitive to all forms of ionising radiation, and records 74 counts/minute. When a 1 cm thick aluminium plate is placed in the gap then the count rate falls to 45 counts/minute. If the source is removed entirely then the count rate remains the same. If the aluminium plate is removed and the source is placed 2 cm from the detector, the count rate rises to 5000 counts/minute. What forms of radiation does the source emit?
A) Alpha only
B) Beta only
C) Alpha and beta
D) Alpha and gamma
The PMT solutions for this paper claim it is alpha only, but I do not see how this can be the case. Surely it is alpha and beta?
The count when there is a 1 cm aluminium plate present (which should block alpha and beta, but not gamma) is the same as when there is no source (therefore the background radiation) - therefore, there is no gamma radiation coming from the source.
When the detector is 6 cm from the source, the count is at 74 cpm. This is 29 cpm higher than the background. At this distance, beta radiation should reach the detector, but not alpha. This suggests the source is emitting beta radiation.
Then when the detector is really close to the source, the count is much higher, suggesting there is also alpha radiation, that could not previously reach the detector.
How, therefore, is it only alpha radiation? Surely there is beta there as well?
Reply 1
TobyAS
OP
3
Nevermind... I've realised I'm kinda being daft. The 29 cpm is probably from a small amount of alpha that has managed to travel the 6 cm. I mean you can't say for certain there is no beta, but I guess the fact that it is such an insignificant amount compared to the alpha makes it unlikely.
Reply 2
14
The 45 cpm is background radiation. At 6cm, 29cpm of alpha radiation from the sorice reaches the detector if there is no metal plate. When the metal plate is placed, then all 29cpm of alpha radiation are blocked bringing the count back down to the background of 45 cpm.
1cm of metal is incredibly thick by alpha radiation standards - the average distance it can propagate in solids like aluminium is on the order of micrometers. Om the other hand, higher energy alphas (8 MeV) have a meaningful chance of reaching the detector 6cm away. So 0.5% of an alpha source reaching 6cm is plausible.
If betas were a significant part of the radiation, you would expect there to be a difference between 6 cm with no radiation source and 6cm with the shielding. But the problem states that in both cases the count is the same as background radiation - in other words, the shield blocked all the radiation.
1cm of metal is incredibly thick by alpha radiation standards - the average distance it can propagate in solids like aluminium is on the order of micrometers. Om the other hand, higher energy alphas (8 MeV) have a meaningful chance of reaching the detector 6cm away. So 0.5% of an alpha source reaching 6cm is plausible.
If betas were a significant part of the radiation, you would expect there to be a difference between 6 cm with no radiation source and 6cm with the shielding. But the problem states that in both cases the count is the same as background radiation - in other words, the shield blocked all the radiation.
(edited 5 months ago)
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