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Air Filter Alpha Spectrometry Report

Air Filter Alpha Spectrometry ReportPosted: 7/31/2011 21:05

Back to Main UCB Air and Water Sampling Page

Back to Air Sampling Results Page

Summary

BRAWM took an air filter sample in mid-April and performed alphaspectrometry on the air filter. We did not detect any Uranium,Plutonium, or Americium-241, and our calculated detection limitsindicate safe levels. Our limits are much higher than the limits setby the EPA in their testing for Uranium and Plutonium. If you wouldlike to skip directly to our limits, please click here.

Introduction

Radioactivity comes in three primary forms: alpha, beta, andgamma decay. A radioactive nucleus will undergo one or more ofthese decays, releasing particles with characteristic energies.

The fission product nuclei that are the subject of BRAWM'smeasurements are beta emitters with associated gamma rays.Beta particles are high energy electrons that do not have discreteenergies — they are released in a continuum with an "endpoint"energy that depends on the nucleus. Gamma rays are photons("particles of light") and have discrete energies. These discreteenergies are fingerprints for a specific nucleus, and detectinggamma-rays is what BRAWM specializes in. For example,Cesium-137 has a strong gamma-ray "line" at 662 keV, and that iswhat we look for in our spectra.

What about alpha decay? Alpha particles are helium nuclei (twoprotons and two neutrons), and they are very similar to gammarays in that they are released at specific energies that arefingerprints of a given nucleus. For example, Uranium-238 has twostrong alpha lines at 4.151 and 4.198 MeV. Since many nuclei

Air Filter Alpha Spectrometry Report | radwatch.berkeley.edu http://radwatch.berkeley.edu/AirFilterResults

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might be alpha emitters but not strong gamma emitters (e.g.,U-238 and Pu-239), one must use an alpha detector to detectthem.

Alpha spectroscopy is difficult, even when compared to gammaspectroscopy. The primary difference is that since alpha particleshave electric charge (+2), even tiny amounts of material betweenthe nucleus and the detector will cause energy loss or completestopping of the alpha particle. So instead of nice, discrete linesyou will find smeared out blobs in your spectrum — or evennothing at all. The more the alpha lines are smeared and blocked,the worse your ability to detect and identify alpha emitters is.Gamma-rays do not suffer from this problem.

Experimental Setup

We placed a piece of filter paper (3M Filtrete) over the nozzle ofthe shop vac used for our air measurements for three days, April16–19, 2011. We filtered a total of approximately 5,500,000 litersof air (5,500 cubic meters). At the end of the collection time, thefilter paper had turned black from the air particles it filtered fromthe air.

Within minutes, we took the filter paper and placed it in front ofand nearly in contact with a silicon PIN alpha detector. Thedetector and sample were housed in a sealed chamber that weevacuated so that the air would not block the alpha particles. Thechamber was covered with an opaque black cloth so that theambient light in the room would not create noise in the silicondetector.

Natural Alpha Background from Radon decay products

Within minutes, there were four prominent alpha lines between 5and 9 MeV. We identified these as coming from Polonium-210 (5.3MeV), Polonium-212 (8.8 MeV), Polonium-214 (7.7 MeV), andBismuth-212 (6.1 MeV), all naturally-occurring isotopes that aredecay products of Radon gas (specifically, the two isotopesRn-222 and Rn-220). It is not surprising to see them.Polonium-214 was the brightest line, but it was only visible in thespectrum during the first few hours. This is easily explained by itsposition in the decay chain of Radon-222 — it is "fed" by thedecays of the beta-emitting nuclides Lead-214 (27 minutes) andBismuth-214 (20 minutes), and it has an extremely short half-lifeitself (164 microseconds). So Po-214 is very bright for the firsthour but disappears when the decay chain is exhausted.


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This plot shows the spectrum from the first three hours, overlaidwith a spectral model for the four isotopes:

This next plot shows the spectrum for 3 days after the Po-214 hasdecayed away. The three isotopes Po-210, Bi-212, and Po-212remain:


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After the first 3 days, the Bi-212 and Po-212 entirely disappear.This makes sense because they are on the Rn-220 decay chainafter the beta-emitting isotope Lead-212 (10.6 hours), so after afew half-lives of Pb-212 this decay chain is exhausted.

What remains is Polonium-210 (138 days), which is "fed" byanother Radon decay product, Lead-210 (22.3 years). So Po-210remains for a very long time. In fact, its activity has been steadilyincreasing during our test as more and more Pb-210 decays intoPo-210. Here is what the spectrum looks like from three days afterthe start until presently:

Searching for Uranium and Plutonium Signatures

Plutonium, Uranium, and Americium isotopes emit alpha particlesat well-known energies, all in the vicinity of the 5.3 MeV line fromPo-210:

Isotope Alpha Energies

U-238 4.1–4.2 MeV

U-234 4.7–4.8 MeV

U-235 4.3–4.4 MeV

Pu-238 5.4–5.5 MeV

Pu-239 5.1–5.2 MeV


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Am-241 5.4–5.5 MeV

So the basic idea is to look at the alpha spectrum at thoselocations and see if there is any excess from those isotopes. Aline, if present, should have the same shape as the alpha linesfrom the naturally-occurring isotopes — i.e., "triangle" shapes withthe rightmost edge at the alpha line energy.

I fit the Po-210 spectrum with a model (red line in the previousfigure), and subtracted the model from the observed spectrum. Itis in this subtracted spectrum that the signatures of the isotopes ofinterest would lurk. Here is the subtracted spectrum, with blacklines denoting the "1 sigma" uncertainties for each bin:

The subtracted spectrum has no clear signatures of any excesscounts. These data appear to be random fluctuations around zero.So now we must go about determining what the detection limit(MDA) of this test is.

Calculating the Limits

To infer what this non-detection means, we calculated an upperlimit on the total activity concentration for these isotopes.

One very important concept is efficiency. Efficiency is the fractionof all emitted alpha particles that are detected by the detector.There are various independent components to it, which I haveconservatively estimated as follows:


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Efficiency Estimate Reasoning

Geometric 3.9%This is the ratio of the area of thedetector to the area of the filter paper.

Left/Right 50%Half of all alphas will travel away fromthe detector, and half will travel towardsit.

Escapefrom filter

0.33%

This is where the largest uncertaintylies. This was estimated by assuming a1 micron range for alpha particles inorganic material, and noting that thetotal thickness of the filter paper is 150microns, with a course mesh coveringabout 50% of the fine filter paper.

Detection 100%Essentially all alphas that strike thedetector should be detected.

Total 0.0064%

Similar to the efficiency is what I will call the "spectral form factor."To calculate the detection limits, we'll select a certain region of thespectrum, such as 3.2–4.2 MeV for U-238. Since an alpha line willspread out beyond this range, we need to know what fractionshould fall in that selected range. Based on the Po-210 line shape,a window 1 MeV wide should yield good sensitivity, but only about50% of counts would be registered. We will use 1 MeV windowsand a spectral form factor of 50%.

Here is some math showing how minimum detectable activity(MDA) is calculated:

Number ofAlphas fromSource

=

Activity Concentration (Bq/L)

× Air Volume (L)

× Counting Time (sec)

× Efficiency (%)

× Spectral form factor (%)

ActivityConcentration

= Number of Alphas from Source


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(Bq/L)Air Volume (L) × Counting Time(sec) × Efficiency (%) × Spectral

form factor (%)

BackgroundNoise

=SquareRoot[ Number of Alphas inBackground ]

MinimumDetectableActivity (Bq/L)

≈

2 × Background Noise

Air Volume (L) × Counting Time(sec) × Efficiency (%) × Spectral

form factor (%)

For our test, here are the relevant data:

Air Volume: 5,500,000 L

Counting Time: 7,259,912 sec

Efficiency: 0.0064% (estimated)

Here are our estimated Minimum Detectable Activities for eachisotope of interest:

IsotopeRange ofspectruminvestigated

Number ofalphas inbackground

MDA(Bq/L)

MDA(aCi/m3)

Years ofbreathingthe air toequaldosefrom oneplaneflight (5millirem)

U-238 3.2–4.2 MeV 3,036<

8.6E-8Bq/L

< 2,300aCi/m3

> 2.6years

U-234 3.8–4.8 MeV 4,354<

1.0E-7Bq/L

< 2,800aCi/m3

> 1.9years


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U-235 3.4–4.4 MeV 3,445<

9.2E-8Bq/L

< 2,500aCi/m3

> 2.4years

Pu-238 4.5–5.5 MeV 5,047<

1.1E-7Bq/L

< 3,000aCi/m3

> 0.7years

Pu-239 4.2–5.2 MeV 5,588<

1.2E-7Bq/L

< 3,200aCi/m3

> 0.6years

Am-241 4.5–5.5 MeV 5,047<

1.1E-7Bq/L

< 3,000aCi/m3

> 0.7years

Comparison with EPA results

While our limits are comparable to the limits we have set forfission product isotopes from Japan, the EPA reached much lowerlimits. The EPA performed several tests for Pu and U in the firstfew weeks after the Fukushima disaster. They issued a report ontheir findings. In San Francisco, there was a detection of U-238,but no detections of U-234, U-235, Pu-238, or Pu-239. Here arethe data, accessed via the EPA query search. MDA was estimatedby taking twice the uncertainty (CSU):

IsotopeResult(aCi/m3)

MDA (aCi/m3)

U-238 14.1±6.3 12.6

U-234 ND 10.8

U-235 ND 8.8

Pu-238 ND 7.0

Pu-239 ND 11.6

So our limits are about 200–300 times higher than the EPA limits.Even though we spent a longer time collecting data, the signal-to-noise is just too poor for us to reach similar limits.

Because of the difficulties inherent in measuring alpha particlesfrom an air filter, the standard procedure is to perform somechemistry on the filter to extract and concentrate the actinides(Uranium, Plutonium, Americium, and others). These elements arethen electroplated onto a metal film and placed in front of a


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detector. Because there is no intervening filter material, thesesamples have much sharper lines and greater sensitivity (i.e.,lower limits) can be achieved.

Since uranium and plutonium have long half-lives, we mayconsider performing other analyses on our filter sample that couldbe more sensitive.

More Information

If you would like to know more about alpha spectroscopy, here area few online references:

EPA Alpha Spectrometry Tutorial

Canberra: A Practical Guide to Successful AlphaSpectroscopy

Canberra: Sample Preparation for Alpha Spectroscopy


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