venerdì 15 febbraio 2013

Radon from Rome, Cesium from Fukushima: environmental radiation and gamma spectroscopy

The detector C12137, at the bottom right in the photo. 
(iaIt connects via USB to the computer for the acquisition
(Italian version here)
The Fukushima accident released large quantities of radioactive materials in the environment, especially iodine and cesium. Iodine decayed in  a few weeks, leaving  cesium-134 and 137 as potential contaminants of the environment and food. A Geiger counter is sufficient to measure the amount of environmental radiation. This detector, however, counts indistinctly each gamma ray that hits it, regardless of the energy of the particles.To estimate the amount of cesium in the environment (and especially in food) is necessary a spectrometer, capable of determining the energy of gamma rays. Since each isotope emits gamma rays of specific energies, from the analysis of the peaks, it is possible to determine the amount  of the various isotopes present.
Recently, Hamamatsu photonics, a company specializing in the development of detectors for space physics, particle and medical devices, has released a portable detector ( C12137) with a crystal to stop the gammas (CsI) and a Silicon Photmultiplier (or a MMPC as they call them) to reveal the energy by measuring the emitted light . The simplicity of this relatively new detector, invented by a Russian scientist, is that it does not requires high voltages, is as small as a pack of cigarettes and you can connect to any USB port. As all  spectrometers,  however, the cost is about 20 times that of a Geiger counter.

The  picture above   shown the radiation in Rome, in a fourth (fifth by Japanese counting) floor apartment: it is 0.25microSv/hour (with peaks of 0.35).
As mentioned, the advantage of the gamma spectrometer, however, is to count, for each decay, the energy of the rays that hit him. In about an hour and then it is possible to obtain a spectrum which describes the type and amount of ambient radiation. To improve the statistics and better highlight the peaks is, however, advisable to wait a while longer. In the figure below you can see how the spectrum in Rome (and in much of Italy) is dominated by radon 222, a noble gas source to the high amount of environmental radiation. Usually the radon comes from the subsoil and from tuff, but in this case, being a flat on the fourth floor, is more likely to come from pozzolanas of construction materials. 

Spectrum of the radiation environment in a home 
of Rome. The peaks of radon 222 are clearly visible. 
Spectrum of an hour equivalent averaged over 7 hours

The figure below compares two spectra  taken in Rome with two acquired in Japan. The value of Rome is the higher (0.25microSv /h), followed by the basement laboratories of Tor Vergata University in Rome(0.10 microSv / h, where, however, there is not much radon), Kokubunji (0.05), and the fourth floor in Wako (0035 microSv / h).Note the almost total absence of radon in Japan (in the onsen, however, may be  higher). The peak at 1460 keV is probably due to Potassium 40 (aka Calium - the same present  in bananas).
Radiation comparison between Italy and Japan. 
The radioactive background is higher in Rome than in Tokyo

The arrow marks the 660 keV regiorn where   the peak of cesium 137 should be located: as is evident there are no measurable quantities of this species above the bottom of the radiation environment. 

Cesium can be detected detected in various hotspots in the region of Fukushima and in soil samples: this topic will be discussed in a next post.

(1) continues
Previous post on this topic in english:

Let's measure Radiation: the radioactivity of everyday objects 放射線を測って見よう (also in Japanese)

Survivalism and the real radiation contamination in Japan