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Safety in the Laboratory

This is a real laboratory. All of the experiments have been designed with safety in mind. Nevertheless, there are certain precautions which you should take at all times; these generally involve common sense.

1. High Voltage

Most of the experiments use normal line voltages (110-120VAC) taken from wall outlets and you should use the same precautions that you use at home, i.e. avoid frayed cables, keep fingers away, etc. In addition, some of the equipment involves the use of higher voltages, up to 2000 VDC. These can pose a higher shock hazard although most have been designed to draw only very low maximum currents. Make all connections with the power source turned off. If you do need to make manipulations near high voltage connections while power is on, it is good practice to use one hand only, taking care to keep the other hand away from any other conductor.

2. Liquid Nitrogen 

Liquid nitrogen, despite being extremely cold, can cause very severe burns. Always wear safety glasses while pouring or transferring liquid nitrogen and do not mix it with any other liquid, e.g. by pouring it down the sink. Let it vaporize naturally. Use the gloves provided if you need to transfer liquid nitrogen from a large dewar. Better yet, get an instructor to do it!

3. Lead 

Lead is poisonous. Lead sheets and bricks are occasionally used as radiation shielding. Handle them with gloves and wash your hands after such handling.

4. Optics

Optical eyepieces should be wiped with isopropyl alcohol before using them.

5. Radiation

Since this is likely to be more unfamiliar to you, we shall discuss this in a little more detail. In this laboratory, there are several experiments which use standard radiation sources for calibration, etc. They pose no particular radiation threat if handled intelligently. However, they can be very much more hazardous if they are ingested into the body. To prevent such an extremely unlikely occurrence, no food will be allowed in the laboratory.

More on Ionizing Radiation

The usual radiations emitted by decaying nuclei were labeled a, b, g by Rutherford before they actually had been identified. Since they are emitted whenever nuclei de-excite, their energies are typical of nuclear level spacings, i.e. a few MeV, about 106 times greater than the energies of visible light emitted during atomic transitions. These ionizing radiations are capable of causing damage to human tissue. Since both a and b particles are electrically charged, they lose energy by ionizing the medium directly. While g-radiation does not directly, its interaction products (electrons) are charged and these ionize.

The a-radiation was shown to consist of helium nuclei which are often emitted in radioactive decay. Typical energies are a few MeV. Since they are heavy and relatively slow moving, and are also doubly charged, they are very densely ionizing along their path. As a consequence they don't travel far in matter: a few cm of air, a sheet of paper, or a layer of skin will stop them completely. For these same reasons, however, a radiation is extremely dangerous when ingested! This is the well-advertised danger arising from radon in the air we breathe. Radon is a noble gas which emits a particles when it decays, as does its daughter products. It has a half-life of about 4 days and it arises from the decay of uranium and thorium isotopes which occur naturally in the earth. Since earth, sand, bricks, concrete and other construction materials all contain these isotopes, radon is ubiquitous. Good ventilation is the best cure. There are no a-sources used in this laboratory.

b-radiation is electrons (or positrons). These electrons have continuous energy spectra (although not always) up to a maximum of 1 or 2 MeV, typically. Since they are highly relativistic, they are lightly ionizing and so travel much further than a-particles, up to about 1 cm in tissue.

g-rays are the electromagnetic radiation emitted during nuclear de-excitation and have energies up to 1 or 2 MeV, typically. These g-rays do not ionize directly but interact via photoelectric and Compton scattering primarily, each of which gives rise to a final state electron, which is ionizing. These secondary electrons have ~MeV energies and ionize as described above. However, the typical interaction length of the initial g-rays is about 10 to 20 cm in water (or human tissue), i.e. they travel this distance before Compton scattering and producing an electron with a range of about 1 cm. These MeV g-rays are therefore very penetrating although they do not produce a high density of ionization. It takes a few cm of lead or iron to significantly attenuate them. Since their intensity falls as the inverse of the square of distance, distance provides the best protection.

X-rays are also electromagnetic radiation but with much lower energy than g-rays, 10s of keV. They interact via the photoelectric effect, have a relatively short range, and can be shielded by a few mm of lead.

Natural Radiation

The earth contains several naturally occurring radioactive isotopes. The principal of these are ²³⁸U, ²³²Th and ⁴⁰K, all of which have half-lives longer than the age of the earth and are present at levels of a few parts per million by weight, typically. ²³⁸U and ²³²Th have decay chains leading eventually to stable lead isotopes, but emit several as, bs and gs in the process. ⁴⁰K emits bs and gs. The charged as and bs are mostly absorbed before they emerge from the ground, floor, walls, etc., but the gs produced within a few cm of the surface have a good chance of escaping. Very roughly, in your home, in your workplace, outside, there is a flux of hundreds to thousands of g-rays/s across any 1m² area.

Cosmic rays are very high energy particles which interact in the atmosphere to give high energy ionizing secondary particles. At the surface of the earth, these consists mainly of muons, which are a type of heavy electron. These are lightly ionizing, like electrons, but are extremely penetrating and produce about the same net radiation dose as the earth's radioactivity.

The unit of radiation dose (for biological damage) is the sievert. 1 Sv is the absorbed energy dose in J/kg x a biological factor (=1 for electrons, = 20 for as).  A useful rule of thumb is that the dosage at a distance of 30 cm from a g-source is approximately 60 CE mSv/hr, where C is the source strength in Curies (1 C = 3.7x10¹⁰ disintegrations/s) and E is the g energy in MeV.  A typical lab g calibration source has a strength of 3 uC (microCurie) and an energy of 1 MeV.  Thus at a distance of 30 cm and over a lab period of 3 hours, the dosage would be about 0.6x10^-3 mSv.  This is quite close to the general natural radiation background level.

The following table is taken from a publication of the American Nuclear Society.  It allows you to make an estimate of your own annual radiation dosage.