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Radiation hazard October 26, 2006

Posted by dorigo in personal, physics, science, Uncategorized.

Ionizing radiation, we have come to know, is a potentially life-threatening hazard.

Radioactivity was discovered in the late nineteenth century, when Bequerel (1896) was studying phosphorescence of different kinds of minerals, and he found that even if kept in the dark, uranium salts would blacken photographic emulsions.

For a while it was thought that radioactivity bore no harm to the living beings – in truth, it was even offered as a revitalizing potion some eighty years ago, in the form of radium and thorium traces dissolved in water. Radithor was its name, and its use brought death to its inventor as well as to many others in the twenties.

Now, after a hundred years of studies of the forces behind radioactive decay, two nuclear bombs dropped on Japan, a few more tested in the pacific killing or injuring nearby fishermen, and other medical evidences, we know pretty well that a radioactive dose has the potential to kill human beings, by causing irreparable damage to cells, modifying DNA, and producing hard-to-heal burns.

We do not know all about the damage of radioactivity to flesh and bones – we know much better the interaction of radiation with matter, since that is important for the design of particle detectors! But what we know gives us limitations to the amount of radiation a human being can be subjected to before it becomes really risky.

Radiation dose to man is measured by a unit called rem . It takes into account the energy released by ionizing radiation as much as the different damage that different kinds of radiation produce on the human body. To give you a scale, during a year on the surface of our planet your body gets a dose of about 360 millirem, something your cells are able to cope with and whose damage are able to repair. That dose comes mostly from radon emitted by rocks, which we inhale, and cosmic rays.

The US Department of Energy allows its workers to get a dose as high as 5 rem per year. It is estimated that few or no effect is apparent on the human body for doses below 15 rem per year.

At Fermilab, radiation hazard is a potential issue. Accelerators produce radiation and the beam can activate (make radioactive) materials that are usually non-radioactive. Moreover, radioactive sources are used widely for calibration of instruments and detectors. To cope with that, a whole division at Fermilab (the Radiological control organization) is in place to keep the risk to users and employees at a minimum.

The facility is owned by the US Department of Energy, but safety being one of the commitments of the lab, the administrative limit is lowered to 1.5 rems per year. Of course, for Fermilab to be able to monitor the amount of radiation its employees receive through their daily activities in the lab, there needs to be a monitoring system.  When employees have to perform work activities in an area where radiation exposure is possible, they have to wear personal dosimeters.

There is a complex policy for the management of radioactive waste – mostly the dust that gets activated by beam products inside the accelerator tunnel, plus other materials used in pumps or magnets. The disposal of radioactive waste is complex and costly, so a lot of care is put to prevent the creation of unnecessary radioactive waste. Moreover, if radioactive waste of unknown origin or composition is created, or materials that are hazardous even if not radioactive are activated, the costs for characterization and proper disposal become huge.

Employees as well as visitors are required to attend a course if they have to work in controlled areas or in places where dosimeters are needed. I just attended such a course yesterday, for the third time in my long career in CDF. It was a useful refreshment of my memory on several issues everybody here should be aware of.

Overall, Fermilab is a very secure environment for radiation. Contamination of individuals is an extremely rare occurrence and it usually boils down to having the subject shower under monitoring, and nobody gets hurt. The lab has to be careful because otherwise the DoE could even decide to shut us down…


1. Georg - October 26, 2006

I though the rem was a mostly obsolete non-SI unit, and the SI unit now being used was the Sievert (Sv).

2. marco - October 27, 2006

yep, the unit to be used should be the Sievert, which is Gy (J/kg) times the quality factor specific for each source of radiation

3. dorigo - October 27, 2006

I agree with you guys, but Fermilab and the DoE are still stuck with the radiation-equivalent-man, a measuring unit still common in the US and UK (my own digital dosimeter also reads rems).


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