Particle Reflectors for Photons

My radio-chemistry professor is an old bloke, and like most old professors is special. In our section on detection he advocated using shielding to increase the count rate of a weak source.  The idea is that the shielding will serve to reflect some of the radiation back towards the detector.  I argued against this, as you will be losing spectra information for each of the collided count as energy will be lost in the quanta of radiation incident.

Four separate geometries were simulated in MCNPX.  The first was a bare HPGe crystal (with the aluminum housing) 10 cm away from a mixed Cs-137 and Co-60 point source. The second (side scattering) was with 1 cm thick lead cylinder encasing the air between the source and the detector. The third (all scatter) was with a 1 cm thick lead  cylinder on the sides, and a 1 cm thick slab below the source.  The final was with a 2" (5.08 cm) thick (all scatter thick bottom) slab below the source and a 1 cm thick side scattering cylinder.  1 cm of lead was chosen because 1.07 cm is the half-thickness for lead (Co-60 source).

X-Z profile of side scattering

X-Z profile of back scattering and side scatter (all scattering)

This also provides cute little examples of solid angle calculations. The simplest fraction of the solid angle the detector subtends is the ratio of the area of the detector to the area of the sphere of radius the distance from the source to the detector is.  For example since the simulated detector is a cylinder along the z-axis with a radius of 2.54 cm, and is 10 cm away from the point source, as is shown below.
\begin{align}
\eta &= \frac{A_{detector}}{A_{Sphere}} \newline
&= \frac{\pi(2.54cm)^2}{4\pi(10cm)^2} \newline
&= 0.016
\end{align}
A more accurate solid angle fraction can be calculated as follows:
\begin{align}
\eta &= \frac{2\pi\left(1-\cos(\theta)\right)}{4\pi}\newline
&=\frac{2\pi\left(1-\cos \left(\tan^{-1}\frac{2.54}{10}\right)\right)}{4\pi} \newline
&= 0.015
\end{align}

Okay, so we do get around a 30% increase in the incident flux with reflection.  This was higher than I expected. In addition, we don't take that much of a hit on our detector response, as shown below. I used  a gaussian energy broadening on the pulse height tally in order to accurate simulate the HPGe response, which is something that I haven't done before and was is really nifty.  What could be super nifty would be to compare this to a measurement, but I don't think that is gonna happen.

Gamma Spectra Simulations

I've tried to simulate the spectra that our detectors see in MCNPX from a Cs137 source (0.6617 MeV), and Co60 (1.17 and 1.33 MeV).  You can observe the Compton Edge (maximum energy of a Compton scattered electron) in both spectra; notice that the Co60 spectra has two compton edges because of the two photons it emits.  You can see the photopeaks, which is caused by when all of the incident energy is deposited in the detector clearly in the Cs137 spectra.

EJ-200 Gamma Response.  Black is Cs137, blue is Co60

GS20 Gamma Response.  Cs137 is black, Cobalt 60 is blue.

The simulated spectra where then compared. The EJ200 is 1/4" thick 2" diameter, while the GS20 is 1" diameter and about 1/8" thick, that is why the EJ200 has a higher tally than the GS20.  You can notice the extra resolution that using a glass scintillator has a much higher resolution (notice the photo peaks that occur at 1.17 and 1.33 MeV in the black, and the absence of them in the the blue).

Co60 Comparison.  The black is the GS20, the blue is EJ200.

The problem is that the count rate's do not match the observed spectra.  However, the shapes are the same.  Gives hope for tomorrow, anyway.

Observed Spectra from GS20.  Green is the Cobalt 60.

Gonna have to trust me that the shapes are the same, since I don't have a spectra readily available on a log ordinate axis.  The channel numbers are analogous to energy, since the channel number corresponds to a voltage, which in turn corresponds to the amount of light collected by the PMT, which in turn corresponds to the energy deposited by the photon.

Really Dude? (Spun PS LiF Film)

We must be reaching the limits of what we can create for new detectors to measure.  We took a rat's nest of 250 micron spun PS fibers with LiF and some fluor and melted them to create the beauty below.  Amazing enough, it gives some counts.

Before Mounting

After Mounting

It reminds me of frosted Wheaties.  The cool thing, I think, is that once it was melted (adding energy) the fibers relaxed to an agglomeration, not a coating (which we had hoped for).