CPM of Gamma in energy range 600-800keV

Found this archived at Berkeley

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Submitted by Anonymous on Thu, 2011-04-28 19:40

• Berkeley Radiological Air and Water Monitoring Forum

Thank you for contacting the U.S. Environmental Protection Agency. We
appreciate your concern. It’s important to know that the U.S. Nuclear
Regulatory Commission has said we do not expect to see radiation at
harmful levels reaching the U.S. from damaged Japanese nuclear power
plants.
The Environmental Protection Agency maintains a nationwide radiation
monitoring system known as RadNet. This system continuously monitors the
nation's air and regularly monitors drinking water, milk and
precipitation for environmental radiation. As of 3:00pm (EDT) April 19,
2011, EPA's RadNet radiation air monitors across the U.S. show typical
fluctuations in background radiation levels. The levels detected are far
below levels of concern.
Data from each fixed RadNet monitor are transmitted to EPA’s National
Air and Radiation Environmental Laboratory hourly. These data include
the gamma radiation measured during that hour. The data are screened
against pre-set values for each monitor to identify unusual readings,
particularly elevated radiation levels. In order to increase sensitivity
for screening the spectrum for small increases in gamma radiation, EPA
divides the gamma radiation measurement data into nine different gamma
energy ranges that collectively cover the energy spectrum where
essentially all of the nuclides of concern will be detected by our
monitors.
When the RadNet computer system detects a reading from a monitor that is
outside the range of background levels typically seen by that monitor,
those data are flagged to be reviewed by a trained EPA scientist. The
review includes obtaining the gamma spectrum stored in the monitor and
evaluating that spectrum for both natural and man-made nuclides.
EPA scientists use a science called gamma spectrometry to evaluate the
complete spectrum of the monitor’s specific calibration to detect the
type and amount of gamma emitting radioactive material at that location.
In this way, EPA scientists are able to determine what specific isotopes
are present in the air at any given time.
For information on the energy ranges for the gamma charts:
The following table shows the energy ranges that correspond with the
gamma charts on our website. Radioactive material from Japan could be
seen in various energy ranges, depending on the specific radionuclide.
Iodine-131 and cesium-137 would primarily be seen in ranges 3 and 5,
respectively, along with other naturally occurring radioactive
materials.
Please note that fluctuations in the gamma readings may be caused by a
number of factors, primarily naturally occurring radioactivity in the
environment. Two of the most prominent naturally-occurring radionuclides
in air are lead-214 and bismuth-214. Natural levels of these nuclides
fluctuate significantly in the environment and their primary gamma
energies are similar to those of cesium-137 and iodine-131. When the
RadNet computer system detects a reading from a monitor that is outside
the range of background levels typically seen by that monitor, those
data are flagged to be reviewed by an EPA scientist. The review includes
obtaining the gamma spectrum stored in the monitor and evaluating that
spectrum for both natural and man-made nuclides.
Energy Ranges
Energy | Gamma Energies
Range | (keV)
Number |
---------+---------------------
1 | Reserved by
| software for
| instrument
| stabilization
---------+---------------------
2 | 100-200
---------+---------------------
3 | 200-400
---------+---------------------
4 | 400-600
---------+---------------------
5 | 600-800
---------+---------------------
6 | 800-1000
---------+---------------------
7 | 1000-1400
---------+---------------------
8 | 1400-1800
---------+---------------------
9 | 1800-2200
---------+---------------------
10 | 2200-2800
As part of the federal government's continuing effort to make our
activities and science transparent and available to the public, the
Environmental Protection Agency will continue to keep all RadNet data
available in the current online database. Please see
www.epa.gov/japan2011 for more information.

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additional information

Iodine-131 with a physical half-life of 8.05 days and an effective half-life in the whole body of 7.6 days possesses its greatest energy peak at 360 keV, a second-ranking peak at 280 keV, a third peak at 638 keV and a fourth peak at 724 keV (Arena, 1971). The lower couple fall into range 3 (200-400 keV) , whereas the higher couple fall into range 5 (600-800 keV) in the RadNet graph.

Gamma spectrum of cesium-137 (Arena, 1971). Counts/channel are plotted on a logarithmic scale versus energy [keV]. Peaks are labelled in MeV. 

By contrast, cesium-137 with a physical half-life of 30.17 years and an effective half-life in the whole body of 70 days emits gamma radiation at 662 keV, falling into range 5 of the RadNet graph. However, backscatter (Compton effect), that is low-energy detector counts produced by incomplete energy transfer between the ionizing radiation and the detector material, contributes a considerable fraction of the total count rate to range 3. Although the precise shape of the spectral curves shown above depends on the radioactivity of the sources and the instruments used for the measurement, the energy peaks remain invariable and representative. Therefore, the spectra may can be employed for the demonstration of principles.

Partial integration of the areas under the spectral curves taking the logarithmic scale of the counts/channel into account suggests that the iodine-131 decay contributes about 91 percent of the total count rate summed over both ranges to range 3, whereas cesium-137 decay will contribute about one third to this range. Therefore, if both isotopes are present in the sample, the count rate measured in range 3 does not exclusively reflect iodine-131 decay, and iodine-131 will also contribute to the count rate measured in range 5, though to a smaller degree than cesium-137. Despite this cross-contamination, a prominent increase in range 3 suggests the presence of iodine-131 and in range 5 that of cesium-137. Furthermore, the EPA provides offline post-hoc data for identified radioisotopes.

Regardless of its low spectral resolution, the real-time RadNet graph may potentially have its uses for the identification of a radiological incident. An increase above the average counts per minute (CPM) measured in ranges 3 and 5 beyond three standard errors of the mean can be considered statistically significant with 95 percent confidence. Mean count rates measured in the past can be queried in the RadNet database. Sequences of up to 400 measurements can be downloaded in a batch.