Post by jsmith on Feb 16, 2015 12:19:58 GMT -8
Initial problem: The corona displays a staggering during the 152Eu runs where the red and green crystals (upstream) count more than the blue and white (downstream) crystals.
Summary: Ryan and I have both been looking into this problem for a bit. Our working evaluation is two-fold: the aluminum source holder was absorbing some low-mid energy gamma-rays (<1.5 MeV) and the 152Eu source also appears to have some kind of additional absorption due to the construction of the source. We have designed a plastic source holder and are testing prototypes to eliminate the aluminum absorption from future calibrations. We also plan on taking some source data with the old source holder, without it, and with the new source holder to pin down exactly the effect of the source holder in order to account for this is data that has already been collected.
More details: I'm attaching my initial evaluation of the problem. Ryan also pulled some attenuation coefficients and did a back-of-the-envelope calculation to determine if the drop in counts we were seeing could reasonably be attributed to the source holder:
The source holder is ~10 cm tall, so at an oblique angle, this amount of aluminum is not unreasonable.
Summary: Ryan and I have both been looking into this problem for a bit. Our working evaluation is two-fold: the aluminum source holder was absorbing some low-mid energy gamma-rays (<1.5 MeV) and the 152Eu source also appears to have some kind of additional absorption due to the construction of the source. We have designed a plastic source holder and are testing prototypes to eliminate the aluminum absorption from future calibrations. We also plan on taking some source data with the old source holder, without it, and with the new source holder to pin down exactly the effect of the source holder in order to account for this is data that has already been collected.
More details: I'm attaching my initial evaluation of the problem. Ryan also pulled some attenuation coefficients and did a back-of-the-envelope calculation to determine if the drop in counts we were seeing could reasonably be attributed to the source holder:
Energy (MeV) μ/ρ (cm2/g)
5.00000E-02 3.681E-01
6.00000E-02 2.778E-01
8.00000E-02 2.018E-01
1.00000E-01 1.704E-01
1.50000E-01 1.378E-01
2.00000E-01 1.223E-01
3.00000E-01 1.042E-01
4.00000E-01 9.276E-02
5.00000E-01 8.445E-02
6.00000E-01 7.802E-02
8.00000E-01 6.841E-02
1.00000E+00 6.146E-02
1.25000E+00 5.496E-02
1.50000E+00 5.006E-02
2.00000E+00 4.324E-02
In your plots, we see a drop from ~600 counts to ~500 counts. Let’s take the worst case scenario and say that all of the gamma rays are 100 keV. We then require
L = -ln(I/I0)/(u/p)
= 1.07 cm worth of aluminium for the gamma ray to pass through. I’m not sure how reasonable this is.
For the 1 MeV photons, we would need 2.97 cm of aluminum for the same attenuation. If we use the 1.07 cm, we see that we would get ~560/600 photons or 94% of the 1 MeV gammas instead of ~83%. However, in doing this I made a bit of an assumption, the photons that are attenuated in any aluminium on the way to the corona are likely to be scattered into a lampshade and increase the lampshade counts.
5.00000E-02 3.681E-01
6.00000E-02 2.778E-01
8.00000E-02 2.018E-01
1.00000E-01 1.704E-01
1.50000E-01 1.378E-01
2.00000E-01 1.223E-01
3.00000E-01 1.042E-01
4.00000E-01 9.276E-02
5.00000E-01 8.445E-02
6.00000E-01 7.802E-02
8.00000E-01 6.841E-02
1.00000E+00 6.146E-02
1.25000E+00 5.496E-02
1.50000E+00 5.006E-02
2.00000E+00 4.324E-02
In your plots, we see a drop from ~600 counts to ~500 counts. Let’s take the worst case scenario and say that all of the gamma rays are 100 keV. We then require
L = -ln(I/I0)/(u/p)
= 1.07 cm worth of aluminium for the gamma ray to pass through. I’m not sure how reasonable this is.
For the 1 MeV photons, we would need 2.97 cm of aluminum for the same attenuation. If we use the 1.07 cm, we see that we would get ~560/600 photons or 94% of the 1 MeV gammas instead of ~83%. However, in doing this I made a bit of an assumption, the photons that are attenuated in any aluminium on the way to the corona are likely to be scattered into a lampshade and increase the lampshade counts.
The source holder is ~10 cm tall, so at an oblique angle, this amount of aluminum is not unreasonable.