Optimal Detector Thickness

As you may recall from the previous post I was investigating the making a layered detector, and I plotted the contribution of a particular film as a function of how far the film was in the detector.  This lead to the idea to ask what would be the best use of material - by making the detector thinner we could avoid some of the low contributions of reactions on the back of the detector.

I held the volume of the detector constant, and adjusted the length and width.  If we make our detector half of what the current He-3 footprint is (which means we need to make them twice as wide) our efficiency goes from 3.1 to 4.6.

Interaction Rate's in a DHS-DNDO Detector

My Master's Thesis is "due" tomorrow, and today I was just finishing up my MCNPX simulations.  And while I should have done this about a month ago, the results are still pretty nifty.  The following figure shows the effect of moderating a Cf-252 source with a 0.5 cm of lead and 2.5 cm of HDPE.  What is neat is that the 2.5 cm of HDPE (lead won't moderate neutrons much because of a neutron cannot impart much recoil energy to the a lead nucleus) is that the spectra is essentially thermalized - the peak is around 0.025 eV, which if you recall is the kinetic energy of room temperature particles predicted by the Boltzmann Distribution.

The detector assembly is composed of a bunch of layers of thin (less than 100 microns) films each separated by acrylic.  The following two plots show the effects of the different layers.  The first layer see's the most neutrons, but doesn't have the largest contribution to the reactions because it only sees them from the front - there isn't much effect of the reflector.  As you go farther into the detecor the number of neutrons crossing a film decrease (it is not because the films absorbs them, only about 20 neutrons are absorbed in all 120 of the films) but rather neutron's escape and scatter out of the detector.

This is probably my favorite plot of my thesis

I looks like I will graduate with a Masters either in May or Summer, and then hopefully (finger's crossed) I can get out of here by May 2013.  Have to do work.

Dead Time

Dead time is the percentage of time that your detector is unable to record events either from material considerations (light transport), electronics (very slow ADC), or online data processing.  In determining the GARRn (see PNNL 18903 if intrested . . .) we measure the response of our films to a mixed spectra.  The additional gamma flux causes a bunch of low energy counts that aren't present in the pure neutron field, which increases the dead time.  This then cases the spectra to shift to lower energies with a larger gamma flux.

As more sources are added the dead time went to ~5%. 

 However, as you can see by the tail, events are still taking place at high channel numbers, they are just not being sampled as frequently.  Kinda neat now the recording of spectra can be thought of a monte carlo sampling. If we increase the LLD from channel 150 to 1,000 then we see that this artifact disappears (mostly).

Higher LLD decreases the dead time, causing a more accurate sampling. No idea why the 6uCi is shifted to the right.

Further in Monte Carlo world, I've been trying to prove the optimal Battleship (board game) game play based on minimizing the discrepancy.  They use this same idea in quasi-random number generators, where the pick the random values in a predetermined sequence which minimizes the region that is not sampled; for example the Halton Sequence is base two on the interval (0 1] is 1/2, 3/4,1/4,5/8,3/8,7/8,1/8, etc.

I'm trying to update to Geant4.5 to use their new constructors (I want to avoid defining my own sensitive detector and related hit classes) so I'm back to trying to get Geant4 to work in Visual Studio.  It really seems to be full proof, but somewhere I am messing up at the final build, probably because I installed my make tools in Program Files, which has a space in it so won't be treated a properly in a relative path.

Optimal Detector Thickness

6Li has a higher chance of detecting a neutron if the neutron is lower in energy; this is accomplished by moderating the neutron, good moderators are water, plastic, or anything with a bunch of hydrogen (a neutron can lose all of it's kinetic energy to hydrogen).  We can then increase our detection effiency by moderating the neutron's that come into our detector.  This is the front thickness.  You also want a rear moderator / reflector to reflect neutrons back into the detector; this is the rear thickness.

The data sets are grouped by rear thickness.  The interaction rate will  asymptotically approach some limit for the rear shield thickness, but for practical purposes it needs to be a few (5 cm or so) to have light collection.

Anyway, thought it was very cool.

Intrisinic Efficiency of GS20

In order to prove that I can simulate our spectra (using the same input file as Martian, but that is another story) I had to demonstrate why  our GS20 had an higher efficiency.  Our spectra is not really a beam; it is more isotropic because of the moderation of the HDPE.  Using the attenuation coefficients published by Saint Gobain, the following figure shows the intrinsic efficiency of a 0.2 cm GS20 detector, with the GS20 perpendicular to the beam. 

Intrinsic Efficiency of GS20

The above is the reason for the stacking of the following curves in the below figures.  What I did was simulate the detector for neutron  beams incident on different angles.  The important part to note is that for high energies (greater than a few hundred eV) the effect of orientation is seen.  This is because the low energies have a high intrinsic efficiency, so small values don't make a large contribution.

Intrinsic Efficiency of GS20 for different detector angles

Gamma Count Rates

Been spending a bunch of time in Gamma Town.  It's nice, you don't have to interact with anybody*.  Shown below is comparing results to simulated values from the total interactions. 

Source of all errors

Alright, so not perfect. But hey, I'm gonna take it.  It explains a lot of the problems we have been having; doubling the distance about doubles the counts.   I think at the father distances photons bouncing around off the table and the lab might also be a little bit an issue. It is also unsettling how the simulated and calculated cross each other.  Now the problem is going to make sure that we can have a repeatable geometry; so maybe we want to place it around 6" away, where 1/2" in each direction only changes the count rate by 12%.  At 1/4", moving the sample by 1/4" changes the counts by 30%.

*Groan worthy, but I'm proud of it.  Gamma's are photons, and are characterized by a probability of interaction, not like electrons which are always experiencing E&M fields.

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).

MCNPX simulations

I recently got cluster access, so now I can simulate much larger systems!  (I also have managed to bring the cluster down twice in about a week . . .).  So what I am doing is simulating the neutron flux and photon flux in our irriadiator and comparing it to how our detector would perform if it was in the DHS configuration.  For the most part, we aren't doing too badly.  Sure that looks like a huge difference between the spectra, but our source is stronger than the DHS.  I was really worried that the DHS spectra would have a different energy structure than ours, but that doesn't look like it is the case.

What is true is that we have a lot more photons crossing our detector than the DHS.  This is a problem - currently we have been subtracting two types of metals with (mostly similar) attenuation to get eliminate the photon response, but since we are using cadmium as the second type of metal we also cut down on our low energy neutrons, which from the above is a large majority of them.

The first energy bin highlights the difference between the neutron spectra

Very different gamma cross sections (and poor statistics)

It does appear, however, that since the cross section for the (n,t) is larger a low energies that our gamma correction by tube subtraction actually will prove to be bothersome.

Tomorrow I go North!