Happy Birthday!

 Happy birthday David!  Birthday cheers!

David and I aboard the Lady Dawnee

Chewing Gum

How come chewing gum doesn't lose its color?  I was chewing on piece of Extra Spearmint, and that stuff was bright green for a least an hour of use - and you know that color isn't natural. I propose that a piece of chewing gum is placed in a ball-milling machine with some sort of saline solution as the lubricant.  After ball-milling for a long time, the lubricant can then be tested from emission / excitation - one would expect to see the wavelength of the gum's color, and this could be then used to get an idea of the amount of color that leaches from the gum. And that's my grocery shopping thought of the day!

Super Cool Energy Deposition

A while ago I did a short post about the positional energy deposition in polymeric films.  I recently ran the simulation for 25 cm thick films, and it is cool that it is possible to see a lot of fundamental physics in the simulation.

Neutron Energy Deposition. (I wish I was good enough such that you could rotate the figure)

First, you can see for very thick films pratically of the energy is deposited in the film (the neutron reaction has a q-value of 4.78 MeV). There is some backscattering lost of the front edge of the film, shown by the number of events in the first thickness bin.  This effect is decreased as one goes into the film. Secondly, you can see the exponential attenuation of neutrons through the film by th linear decrease in the frequency of counts as you go through the film at 4.78 MeV -  just taking the slice at 4.78 MeV.  This decrease is linear on a log scale, and therfore follows an exponential attenutation.  From this it would then be possible to esimate the macroscopic absoribition cross section, which determines the rate of attenuation in the medium. Pretty cool, huh?

Compton Scattering & Secondary Electron Kinetic Energy Distrbution

Every once and a while it is good to do basic physics again. This problem was for the validation (again) of my GEANT4 code that simulates the energy deposition and distribution of secondary electrons in a thin film in order to make sure that I am grabbing the right particles / using the correct tracking.

The physics involved are the energy of the Compton scattering of the photon
 \[E_e= E - \frac{E m_e c^2}{m_e c^2 + E (1-\cos\theta)}\]
and the Klein-Nishma scattering cross section
\[ d\sigma = r_0^2 f(\theta)\sin\theta d\theta d\phi \]
where
\[f(\theta) = \frac{1}{2}\left(\frac{E'}{E}\right)^2 \left(\frac{E'}{E} + \frac{E}{E'}-\sin^2\theta\right)\].
After integrating out the phi component it is possible to arrive at
\[\frac{d\sigma}{d \theta} = 2\pi r_e^2 f(\theta) \sin \theta \]

Then taking the derivative of the electron kinetic energy allows for the relationship between the scattering probability and the electron kinetic energy.
 \[\frac{d\sigma}{dE_e} = 2\pi r_e^2 \sin \theta f(\theta)\left [ \frac{1+\frac{E}{m_e c^2}\left(1-\cos\theta \right)^2}{E^2 \sin \theta} \right ]\]

The difficult part was in correctly sampling the distribution, for this I just used a simple rejection method in which the probability of an energy is proportional the the scattering angle of that energy as the PDF (shown above) would be difficult to invert. For this a python script was written. The first part was to make sure that I could reproduce the Compton Scattering cross section; i.e. make sure that my math was correct (Hey, I had to do some fancy chain rule stuff and it has been awhile).
 

Scattering Cross Section as a function of Photon Energy (Co-60)

Finally, I sample from the above distribution and binned it according to a logarithmic energy binning, as shown below. The comparison to the GEANT4 data is also shown as the final figure.

Klein-Nishma Electron Kinetic Energy Spectra

Pretty Good, but I don't have the peak slightly below 1 MeV. 

After all of this it is kinda depressing - I spent so much time just making sure I was right that the simulation code wasn't really worth the effort if in two days you can do a quick analytical check - but hey, it is pretty sweet that I can reproduce things.

Golden Age of Science (Operation Teapot)

I don't think any introduction is necessary for this*(AD611311) - You nuke some brews and then drink them!  Science! I have reproduced sections of the summary below:

"In summarization, it was found that commercially package beer and soft drinks in cans or bottles will survive the effects of the blast overpressures such as found at 1270 ft from GZ on Shot II. Nominal losses were occasional primary by flying missiles, the crushing of structures, or dislodgment from shelves. .... Some flavor change was found in the beverages, more in beer than in soft drinks.  However, the alterations may well be considered as equivalent in most respects to "aging" and were not found to detract from the potential usage of these beverages for emergency supplies of potable water".

*I like this guys blog, http://nuclearsecrecy.com/blog/

Spring Time in Tennessee!

My tulips and daffodils planted at Ijams actually lived! I planted them in early winter in order to spruce up a brown field site (essentially the rubble pile from the UTK student union excavation).

Originally the site was supposed to be a playing field, but I think the amount of rebar (some county worker must have really ruined his spindles when he tried to mow it) in the dirt inhibited it, and now it just has crushed stone over it, so probably turning into a parking lot.

I would also like to take this time to present my ankle biter of a rock wall.

I'll be the first to admit that it's not some of my better masonry work (I'm not David), but it's serviceable. I should have really looked around more for better rocks - Ijams is an old quarry site so I probably could have afforded to be picky.  Hopefully next week I'll fill behind the wall and plant some native stuff flowers.  The idea is to guide people along the trail that leads to quarry kilns.

Positional Dependance of Energy Deposition

Often times I find myself making vague arguments about the influence of an neutron interaction position and it's energy deposition.  For example, if an interaction takes place "close" (say 10 microns) very little of the energy from the neutron interaction will be deposited in the film as the range of the alpha and triton are both in the 30-50 micron range and will thus leave the film. However, these are very hand-wavy arguments, so I quantified them with a GEANT4 simulation, bining the postion of first interaction and the energy deposition resulting from that interaction.  The detector is a cylinder with the impingent neutrons along the axis of the cylinder; thus it is axial symmetric and the only dimension of intrest is the z.

The following plots (however badly made; I'm still trying to figure out how to nicely plot things in ROOT) are the results of these simulations.

25 micron neutron - most of the energy depostion comes from the alpha, which has a shorter range than the triton

250 micron film neutron.  Can observe that interactions near the edges deposit less energy, are are far less likely.

2,500 micron (2.5mm) neutron. Essentially all of the energy is deposited in the film.

The gamma probablities of energy depsotion are pretty flat even for very thick films, with most of the interactions depositing very little energy, as expected.

10,000 micron (10 mm) film Co60

Incidently, if anyone knows how to space the axis label from the axis in ROOT, please let me know.

Best Research Group Ever!

Our group was nominated for the 2013 multi-disciplinary research group award! Now we just need commemorative t-shirts!

APS Verification