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Monday, August 27, 2018

Americium-241 gamma radiation from smoke detectors for calibrating various radiation monitors

Below: 59.5 kev gamma radiation from a household Am-242 type smoke detector, measured using Nuclear Enterprises Ratemeter RM5/1 with a Bicron G2LE gamma radiation scintillation NaI probe set to 1100 volts potential. The probe contains a 1mm thick, 5cm diameter NaI crystal designed for detecting low energy gamma rays (10 kev upwards), with a 7mg/cm2 window. This stops alpha particles but allows most 59.5kev gamma rays to enter.  Natural background is 14 counts/second.

Other videos below show the 0.0595 MeV Am-241 gamma radiation dose rate on a calibrated Thermo Eberline FH40 digital dose rate meter, and the effect of 1 mm of steel in reducing the gamma intensity from 59.5 kev Am-241 by a factor of 6, and the variation in intensity with distance, in comparison to various naturally radioactive rock, radium clock dial, thorium gas mantle, and other items for approximate proxy calibration purposes. This Am-241 low-energy gamma radiation is fairly similar to the low-energy gamma rays from U-237 and Np-239 which predominate in fallout during the first two weeks after detonation of U-238 tamper/pusher fission/thermonuclear weapons, as discussed in detail in previous posts on fallout. Contrary to populist civil defence assumptions of 1.25 MeV Co-60 gamma rays for protective factor calculations, the declassified mean gamma ray energy measured by gamma ray spectrometer for close-in fallout from 3.53 megaton thermonuclear shot Zuni at Bikini in 1956 was about 0.2 MeV at 10 days after detonation.  It's very easy to shield.

This post is also a good place to include some useful information about natural environmental radiation.  In the 1970s, the nuclear reactor health physics staff at Harwell, UK, very sensibly put together simple radioactivity demonstration kits to reduce fears about contamination by showing how low the "nuclear pollution" is compared to natural rocks that contain small amounts of thorium-232 or uranium-238.  An example which someone is selling on ebay now is shown below, accompanied by the very interesting instruction sheet:

This kit contains 9 natural and artificial radioactive samples with a Mini Instruments Mini-Monitor geiger counter to allow people to compare the radioactivity level from the different samples.  Included in the kit are various natural radioactive samples from the beaches and mines in Cornwall and Devon, radioactive consumer items like a thorium gas mantle and a radium-zinc sulphite luminous dial clock, and samples of radioactive soil downwind from experimental nuclear reactors at Harwell, and also mud from flats at the Ravenglass Estuary, the most highly contaminated area south of the nuclear power plant, Sellafield.

Dried Ravenglass Estuary mud now contains about 1000 Bq/kg of the alpha and 0.059 MeV gamma emitter Am-241, 300 Bq/kg of Cs-137, and 600 Bq/kg of plutonium-239 and -240.  A single household smoke detector (not included in the kit) contains 37,000 Bq of Am-241 (1 microcurie).  (Data source: REFE-21, Radioactivity in Food and the Environment, 2015, online here, published by the UK Environment Agency and other goverment departments.)  Although 1000 Bq/kg of Am-241 in Ravenglass Estuary mud sounds impressive, you need 37 kg of the dried mud to obtain the same amount of Am-241 as contained in a single domestic smoke detector!

Britain's most heavily artificially contaminated public area actually has trivial radioactivity compared to naturally radioactive hotspots (from RIFE-21, UK Environment Agency).

Above: calibrated Thermo Eberline FH40F2 (serial number 9225), responding to 0.059 Mev (59 kev) gamma rays from a 1 microcurie Am-241 smoke detector source.  Contact gamma dose rate indicated is 0.80 micro Sieverts/hour (which is 8 times the natural background in London), which should allow these gamma emitting smoke detector radioactive sources to be used conveniently for checking the calibration of dosimeters and various kinds of radiation meters, if the instrument is energy compensated to include 59 kev (the FH40F2 is marked, as seen above, to be valid from 45 kev to 1.3 Mev).

Geiger counters will respond to the 59 kev gammas from Am-241 provided these gamma rays are not shielded out by the probe housing (or the side wall of the G. M. tube itself).  Am-241 emits several lower energy gamma rays, not purely 59 kev which is the strongest line in its spectrum, so the experimental finding that 1 mm of steel reduces the Am-241 gamma flux by a factor of 6 applies to this composite spectrum.  The Nuclear Enterprises PCM5 with a DP2R alpha-beta probe, with a paper or plastic cover to stop alpha particles, treats Am-241 gamma rays like low energy beta particles because the Compton electrons produced by low energy gamma rays in the perspex phosphor have typically half the energy of the incident gamma rays, so are similar to low energy beta particles from sources like C-14 or tritium.  These gamma rays are unable to reach the bone marrow due to shielding by intervening tissue and bone, like alpha and beta particles externally.

The half-life of Am-241 is 432.2 years, so varies much less in storage than the usual Co-60 source (only 5.27 years half life).  In addition, Am-241 doubles as an alpha radiation source for testing sensitive contamination meters.  Because both the alpha and gamma emissions are easily shielded, they do not require any special storage (any more than smoke detectors in homes).