Safety in 3rd  Year Physics Laboratory

1 Nuclear Radiation Safety

1.1 Defnitions

1.1.1 Strength of Radoactive sources

The strength of a radioactive source is measured in the number of disintegrations per second. One Becquerel (Bq) is one disintegration/sec. A more common unit is the Curie (Ci). 1Ci = 3.7 · 1010 disintegration/sec. More offen used quatities are mCi and µCi meaning 10-3 and 10-6 of a Cu, respectively.

It should be remembered that a radioactive isotope may emit more than one radiation for each disintegration. For example, the
60Co isotope emits one β and two γ radiations for each disintegration.

1.1.2 Exposure

When radiation from radioactive sources interacts with matter it deposits energy through ionization. Radiation is causing one Roentgen of exposure if it creates one Coulomb of electrons and ions in one Kg of air. This causes the release of an energy of 87.8 erg per gram of air. This quantity is directly measured by radiation monitors like the Geiger Counters.

Other units of exposure are the Gray (Gy) defined as the radiation that releases 1 Joule of energy into 1Kg of material. The Rad is defined as the radiation that releases 100erg of energy into 1 gram of material. 1Gy = 100Rad.

The dose is a measaure of the biological effect of the exposure and takes into account the fact that different kinds of radiation may have different effect on the human body. The “Roentgen Equivalent Man(REM) is the product of the exposure measured in Roentgen by a ”Radiation Weighting Factorthat describes the biological effects. For x, β and γ rays this factor is 1, and for α particles its 10. It should be remembered that β and α particles are usually stopped in the source material and if they get out (of very thin sources) they are stopped in a few cm of air. So this radiation is dangerous only if one actually touches the source.

Another unit of dose is the Sievert (Sv). 1Sv = 100Rem.

1.1.3 Neutrons

Neutrons do not cause ionization directly. Their interaction with matter is through nuclear reactions which may result in ionizing particles. Neutrons require their own badges and monitors. In our laboratory neutrons are emitted from the 252Cf source in the nuclear fission experiment. The intensity of this source is extremely low: less than

1µCi and the neutron radiation is about 0.3µ Rem/hr at 1 meter distance. Still, People should not enter this experiment room without the regular and the neutron badges.

1.2 Dose

1.2.1 Allowed Dose

The offcially allowed doses are as follows:

  • People having no radiation control: 100 mRem/year.
  • For people wearing radiation badges: 1000 mRem/year.
  • For pregnant women wearing badges: 500 mRem/year.
  • For people wearing badges and are under medical control: 5000 mRem/year.
  • The average dose from background radiation is about 360 mRem/year, about half of it from inhaling Radon.

1.2.2 Typical Doses

A radioactive source emitting γ radiation with energy of about 1 MeV and having a strength of 1 mCi will produce a dose of about 6 mRem/hour at a distance of 30 cm. The radiation intensity falls as the square of the distance.

Typical strength of radioactive sources used in the 3 rd year physics laboratory are:

    The strongest source is in the "Compton Scattering" experiment: 100 mCi 137Cs.
    It is kept always inside a lead barrel with only a narrow hole which is opened only during experiments. Do not stand in front of the hole.
  • The sources used in the “Mössbauer effect”: 119 SN m and 57Co have ~ 1mCi strength
  • A 170 µCi 60Co source is used in the γ - γ angular correlation experiment.
  • All other sources are weak, several µCi.

All the sources are kept inside lead shielding except when being used in an experiment. The general radiation level in the laboratory is the normal background radiation level.

2 Other Safety Precautions

2.1 Lasers

Laser light can be very dangerous to the eye, causing irreversible damage and even blindness. Therefore:

  • Do not look into the laser beam, whether visible or not, even if it is being transferred through an optical fiber.
  • Scattered laser light may also be very dangerous. Always wear protecting glasses while working in the vicinity of a laser.
  • Do not enter a room where a laser is in operation, as indicated by the alarm light. If you must enter, wear protecting glasses first.

2.2 Liquid Nitrogen and Helium

These very cold liquids cause burns if they form contact with the skin. Drops may splash from an open dewar. Do not touch materials (pipes, etc.) that are being cooled with these liquids. Also, wear protective glasses when using dewars and stay away from them otherwise.

2.3 High Voltage

High voltage (1000-2000 Volt) is applied to photomultipliers. Be careful when making connections or when holding a photomultiplier assembly. In particular, avoid touching the base and wirings.

2.4 Lead

Lead bricks (or in other forms) is used in the laboratory mainly to shield from radiation.
Remember that lead is poisonous. Wear gloves when handling it and wash your hands or any other body part that may have come in contact with lead.

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