Category: Sciency things

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Astrophotography on Astronomy Cast

One of my favourite podcasts to listen to is Astronomy Cast. I’ve even had the fortune to see the recording of some episodes at Dragon*Con and in a Google+ hangout.

After my recent experiments with photographing the night sky I noticed the two most recent Astronomy Cast episodes were about astrophotography.

Being an astro geek in a former life, astrophotography has been something I’ve wanted to do for a long time now. I also know it can be an expensive hobby.

Astronomy Cast Episode 239 talks about the gear needed: camera, lenses and tripod. Good discussion on the kinds of equipment needed, plus some very useful links in the show notes. They even talk about using webcams attached to telescopes.

Episode 240 discusses astrophotography techniques. Exposure settings, image stacking, a little bit about detector characteristics, filters.

Good info in both episodes. Next episode is supposed to cover image processing.

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Energy Imparted

The research I’m doing for my PhD is taking a bit of a detour from the original plan to look at scatter radiation. Now I’m going to be studying patient dose from an “energy imparted” point of view and the work on scatter radiation is moving to a secondary role.

“Energy imparted” is a relatively new term to me, but now that I’ve done some literature searching and reading up on it, it turns out to be a very familiar quantity.

The absorbed dose to a volume of matter is defined as D = E/m, where E is the amount of energy deposited and m is the mass of the volume.

Turns out E = energy deposited == energy imparted. Duh. lightbulb

More later as I continue reading.

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Dealing with MCNP mesh data

MCNP mesh tallies generate a lot of numbers to deal with. It all gets written to a formatted text file. Depending on the size and resolution of your simulation, you can easily end up with GBs of data in a single text file.

The first simulations I did, I decided to stuff it all into a database and use Excel to pull out the bits I wanted. It’s a little clumsy and somewhat cumbersome but it works.

As I work with MCNP more, I’ll probably come up with better ways and find better tools to help deal with the data. For now, this is the snippet of PHP code I wrote to stick the data into my DB.


// Mesh simulation data format
// '   Energy         X         Y         Z     Result     Rel Error     Volume    Rslt * Vol'
// '  1.000E+36   -98.000   -98.000   -94.000 8.44128E-19 4.58915E-03 6.40000E+01 5.40242E-17';
// Open the file for reading
if ($fh = fopen($mesh_file,"r")) {
    // Read file line by line
    while ($mesh_line = fgets($fh)) {
        // Split the data and insert into the database
        if ($c=preg_split("/\s+/",$mesh_line)) {
            $x=(float)$c[2]; // x
            $y=(float)$c[3]; // y
            $z=(float)$c[4]; // z
            $result=(float)$c[5]; // result
            $rel_err=(float)$c[6]; // rel error
//          $vol=(float)$c[7]; // volume
//          $result_vol=(float)$c[8]; // result_vol
            $res =& $dbh->query("insert into MeshData 
                (sim_id,x,y,z,result,rel_err) values ($sim_id,$x,$y,$z,$result,$rel_err)");
            if (PEAR::isError($res)) {
                echo ($res->getMessage().' - '.$res->getUserInfo());
            }
        }
    }
}
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Backscatter scanner simulation

Threw together this MCNP input file to work out the depth dose curve for a very simplified back scatter scanner. Since I don’t have enough details to model exactly what’s going on, I made a lot of simplifications regarding the geometry.

I’m not terribly concerned about creating a detailed model of the scanner. This is all just back of the envelope stuff. I just want to get some order of magnitude estimates that will give me an idea of how the dose would be distributed.

It’s not complete yet. I still need to work on the source definition and set up an appropriate x-ray spectrum.


Backscatter screener simulation
c Simulation to look at the depth dose curve for one of those
c airport backscatter screening machines
c The "passenger" is modeled as a rectangular slab of water
c 1.8m tall by 0.5m wide x 0.3m thick
c Souce is modeled as a planar source 1.8m tall by 0.5m wide
c based on the assumption that these backscatter machines
c operate using a scanning line source (which seems to be
c a reasonable assumption)
c --------- Cell cards -------------------------------
1     2 -1.0       -1     imp:p=1       $ "Passenger"
2     1 -1.205e-3  1 -2   imp:p=1       $ Air around passenger
c --------- Surface cards ----------------------------
1     rpp -30 30   -15 15   -85 85      $ "Passenger" 60cm x 30cm x 170cm
2     rpp -250 250 -250 250 -250 250    $ universe, 500 * 500 * 500 cm^3
c --------- Data cards -------------------------------
mode  p
rand  gen=2                   $ Use l'Ecuyer RNG, larger random number space
c --------- Source Definition ------------------------
c Define a planar source emitting 50 kVp photons located 150 cm away from
c the water slab
sdef  par=2 x=d1 y=150 z=d2 erg=d3
vec=-1 0 0 dir=1
si1   h -30 30
sp1   d 0 1
si2   h -85 85
sp2   d 0 1
#     SI3     SP3
c 50 kVp spectrum goes here
nps   1e9
c --------- Tallies ----------------------------------
c Mesh tally inside water slab
fmesh4:p geom=xyz origin=-30 -15 -85
imesh=60   iints=60
jmesh=30   jints=30
kmesh=170  jints=170
fm14 1.60651e-5  2 -5 -6          $ mesh tally in uGy per source photon
fc14 Water slab dose in uGy/source photon
c --------- Materials --------------------------------
c Dry air at sea level - Mass density = 0.001205 g/cm^3
m1    6000   -0.000124
7000   -0.755268
8000   -0.231781
18000  -0.012827
c Water - Density 1 g/cm^3
m2    1000   -0.111898
8000   -0.888102
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Backscatter hubbub

There’s been a lot of hubbub and commotion in the news recently about the backscatter security scanners (Advanced Imaging Technology) being introduced at airports. You’ll hear all kinds of claims from both sides about them, some partly true, mostly hyperbole and FUD.

One of them involves the radiation dose to the skin. There are claims that the energy is deposited mostly in the dead layer of the skin, or that the radiation dose is very minimal (from the reports I’ve read, it’s on the order of a few μSv). On the other side you have claims that since all of the dose is deposited in the skin, the risk of skin cancer is greatly increased (not entirely accurate and a bit of an exaggeration at the doses involved).

While I was skimming through all of these discussions, the thought occurred to me that I could use MCNP to do a quick “back of the envelope” simulation to see just how the radiation dose might be deposited. Although I don’t know the details of how the components of these scanners are arranged, I think I can make a few reasonable assumptions based on what I’ve read so far. A “passenger” could be modeled as a rectangular slab of water and assuming a scanning beam is used, the source could be modeled as a uniformly emitting plane source. This would let me determine depth dose curves pretty easily.
I’ll have to do some literature searching to see what else I can find out. I’ve heard of papers where people have measured the doses but I think those have been limited to just measuring the radiation output without looking at dose distribution.

This could be an interesting little distraction.

Update: In the TSA Reading Room you can find a moderately redacted analysis document prepared by the Johns Hopkins University Applied Physics Laboratory that describes their measurements of the radiation output from an early version of one model of backscatter scanner.