Experiments with gamma rays

Inverse Square law with $\gamma$ rays

$\gamma$ rays are high-energy photons. They can originate from the spontaneous emission of a nucleon (proton or neutron). Using such relaxations from Cs atoms (in fact Ba atoms following a $\beta$ transition of Cs), we will address the following particular topics surrounding the use of radiosources:

• the different types of radiation and their main properties, including the existence of background radiation.
• the randomness of radioactive emission and its associated statistical aspect.
• the inverse square law for uncollimated $\gamma$ radiation.

Safety issues

The experiment involve a radioactive source (shown left) and specific safety procedures must be adhered to. We expect maturity and good behaviour from the students. Eating/drinking/applying cosmetics is not allowed during the demonstration and following the staff instructions regarding safety issues and practices in the laboratory are mandatory at all times. Safety instructions will be explained before the demonstration, in particular the procedure to follow in case of an emergency (e.g., fire alarm).

Transfer of the radio-source from its storage locker will be made by the university radiation protection officer. The demonstration will be conducted in a properly equipped and maintained laboratory (in the Rosalind Franklin building). The demonstration will be carried out at all times by a qualified Physicist, with a Diploma of Advanced Studies in Corpuscular Physics and experience in overlooking laboratory sessions with University students handling similar radiosources. It will use a Cs-137 370kBq sealed source, from the last generation in design for safe handling, experimentation, storage and security. Sealing means that the radioactive material is permanently fixed in a capsule designed to prevent release and dispersal of the radioactive material under the most severe conditions which are likely to be encountered in normal use and handling. Spreading of Cs is the most dangerous aspect of this common radioactive element, that is one of the most common waste in nuclear incidents (Tchernobyl and Fukushimas) or contaminating agent following nuclear-weapon tests. The sealing of our source means that Cs is confined and that the risks associated to its dispersion in the surroundings are eliminated. Students should maintain at a distance from the setup (a meter) during the demonstration, in particular, they should not be in contact with the source itself or its support. They will be given the opportunity to participate actively and taking data, without having to interact with the radiosource. The exposure of students and staff will be kept considerably below the legal dose limits (much less than is delivered by ambiant radioactivity).

Part of the demonstration will involve a presentation and discussion of the risks associated to radioactivity, which either tends to be feared to an irrational degree or on the contrary simply ignored. Teaching about ionising radiations helps people to develop balanced attitudes to the subject, neither blasé nor apprehensive.