Radiation Safety

Principles of radiation safety

It is essential for those considering use of radioactive materials to have a reasonable understanding of the concepts of radiation in order to apply the University local rules efficiently.

·         Doses from Radiation

There are four types of ionising radiation.

-         Alpha particles

-         Beta particles

-         Gamma photons

-         Neutrons

Radioactive Decay
Radioactive nuclides (also called radionuclides or radioisotopes) can regain stability by nuclear transformation (radioactive decay) emitting radiation in the process. The radiation emitted can be particulate or electromagnetic or both.  The various types of radiation and examples of decay are described below.

Alpha
They have a mass and charge equal to those of helium nuclei (2 protons and 2 neutrons). Alpha particles are emitted during the decay of some very heavy nuclides (Z> 83). They are also doubly charged and thus highly ionising therefore they slow down very quickly and deposit a large amount of energy in a short distance. Their range is afew mm of air or a very thin layer of paper. Also they are only an internal hazard.

• Alpha particles are the least penetrating of ionising particles.
• Alpha particles cannot penetrate human skin and are easily stopped by protective clothing.
• Alpha particles are stopped by 5cm of air or a thin layer of paper, water, clothing or dust.
• Apha emitting radioactive materials are very harmful if inhaled, swallowed or enter the bloodstream through an open wound.

 

Beta
Beta particles are more penetrating than alpha particles. They are emitted from the nucleus and have a mass equal to electrons. Betas can have either a positive charge or a negative charge. Negatively charged betas are equivalent to electrons and are emitted during the decay of neutron rich nuclides.Positively charged betas (positrons) are emitted during the decay of proton rich nuclides.

• Beta particles will penetrate the skin and if beta-emitting material is left on the skin for a long period the dose may be sufficient to cause skin burns.
• Beta particles will penetrate up to several millimetres of plywood and several metres in air.
• Beta-emitting radioactive materials are harmful if inhaled or ingested.

 

 

Gamma
Gamma (also called gamma rays) are electromagnetic radiation (photons). Gammas are emitted during energy level transitions in the nucleus. They may also be emitted during other modes of decay. They are predominantly stopped by one collision. Unlike beta particles they do not lose energy through matter. The number of photons will decay exponentially with distance – in a similar fashion to half-life (see below).

• Gamma radiation will penetrate may centrimetres of human tissue and can give a radiation dose to all organs of the body.
• Gamma radiation will penetrate many metres in air and the higher energies will penetrate several metres of concrete. High-energy X-ray have similiar penetrating properties.
• Gamma-emitting materials are harmful as external sources as well as being harmful if inhaled or ingested. These sources may also emit alpha or beta sources.

 

Neutrons
For a few radionuclides, a neutron can be emitted during the decay process.

• Neutrons can penetrate all through the body and so irradiate all organs of the body.
• Neutrons can penetrate many metres of air and several metres of concrete.
• Very few nuclides emit neutrons; it is rare for a radionuclide to emit only neutrons.

 

X-rays
X-rays are produced when a fast moving beam of electrons is made to collide with a heavy target such as tungsten. The x-rays are produced when the electrons decelerate - a process known as bremsstrahlung. X-rays are packets (photons) of electromagnetic energy. They have a high energy given by the accelerating voltage in the x-ray set. X-rays and gamma rays are both units (quanta) of elctromagnetic energy and only differ in their method of production. X-rays are produced by processes (such as bremsstrahlung) outside of the nucleus. Gamma rays are produced by processes within the nucleus. Bremsstrahlung must be considered when using large activities of high beta emitters such as P-32 and S-90.

Characteristics of radioactive decay

The decay of a radioisotope can be described by the following characterisitics.

1. Half-Life

The half-life of a radionuclide is the time required for one-half of a collection of atoms of that nuclide to decay. Decay is a random process which follows an exponential curve.

2. Energy

The basic unit used to describe the energy of a radiation particle or photon is the electron volt (eV). An electron volt is equal to the amount of energy gained by an electron passing through a potential of one volt. The energy of the radiation emitted is a characteristic of the radionuclide.

When a person absorbs ionising radiation, the radiation dose is a measure of the energy absorbed. The dose is used to express the dangers of exposure to radiation. The dose due to radiation exposure can be represented by a single quantity, the effective dose, with the aid of a special unit, the Sievert.

Types of exposure to ionising radiation:

-People can be exposed to irradiation by an external source. In this situation radiation material is sealed in the source capsule so that it cannot enter the body. Therefore, any effect is due only to the ionising radiation permeating the source capsule. The effect stops with the capsule is removed.

- People can be exposed if the radioactive material were to escape from the capsule. In this case it would cause contamination and there is a risk that persons exposed to the contamination may suffer an intake of radioactive material by inhalation or ingestion. The material then becomes an internal source. If the conditions of the work, such as the airborne concentrations and breathing rates, are known, the internal dose may be calculated.

Radiation Units

• Activity is a measure of the number of disintegrations from a sample/unit time.

The old unit was the Curie (Ci). 1 Curie is that quantity of radon that is in radioactive equilibrum with 1 gram of radium. 1 Curie is equal to 37 Billion disintegrations/second. The new unit is the Becquerel (Bq) which is equal to 1 disintegration/second.

• Dose is the measure of the amount of radiation a piece of material has received.

The old unit was the rad. The new unit is the Gray (gy) which is the equivalent to 1 joule/kilogramme or 6.2 MeV / kilogramme. 1 Gray is equal to 100 rads. However, this takes no account of the amount of matter and how it interacts with matter.

• Dose Equivalent is a measure of the amount of radiation allowing for radiation types.

The old unit was rem. However, the new unit is the Sievert (Sv) which is equal to 1 Gray x Quality Factor. There is a specific quality factor value depending on the type of radiation as shown in the table below.

 

Radiation Type	Quality Factor
Alpha	20
Beta	1
Gamma	1 (normally)
Neutrons	10-20

 

Summary of Units

Quantity	New SI Unit	Old Unit
Activity	Becquerel - Bq (1disintegration s^-1)	Curie - Ci (3.7 x 10^10 dps)
Absorbed Dose	Dose Gray - Gy (1 J.kg^-1)	Rad - (0.01 J.kg^-1)
Equivalent Dose	Sievert - Sv (Gy x Quality Factor)	Rem - (rad x Quality factor)

Some useful conversions

Dose	Activity
1 Sv = 100 rem	1 uCi = 37 kBq
50 mSv = 5 rem	1 mCi = 37 MBq
200 uSv = 20 mrem	1 kBq = 27 uCi

Protective actions against radiation

Working safely with radioactive material and radiation requires the strict application of guidelines and rules. Advice must be sought from the University Radiological Protection Officer and the local rules must be followed on aspects of radiological protection.

External Source

When the source is contained, it may be possible to view it and handle it from a distance or the protection afforded by a shield may be necessary. The hazard is proportional to the activity of the radioactive material involved in the source. Be sure to reduce the duration of the exposure and increase the distance from the source.

All possible steps should be taken to provide containment for the radioactive material and to reduce the spread of radioactive contamination. If this cannot be done, it is essential to provide protective clothing and an appropriate respirator.

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Protection from External Radiation

One of the most important principles in radiation protection is the ALARA (As Low As Reasonably Achievable).  This can be accomplished by:

(a) Keeping time to a minimum:
The less time you remain in a radiation field, the smaller the dose you receive.

Dose = dose rate x time

-         Dose is proportional to time.

-         Practice using ‘dry runs’.

-         Deftness/confidence.

(b) Keeping distance to a maximum:
The dose rate for most gamma and x-radiation varies with the inverse square of the distance from a "point" source. Therefore, the farther you position yourself from a source of radiation, the smaller the dose you receive.

Dose rate = dose rate at 1 meter/ (distance)2.

For example, doubling the distance from a radiation source will result in 1/4 the exposure in the same amount of time. The practical implementation of this principle is in the use of remote handling devices such as forceps, tongs, tube racks, etc. to minimize direct contact with sources and containers. Even a small increase in distance can result in a dramatic decrease in dose rate.

- Inverse square law (dose α 1/d2)
e.g. doubling the distance will reduce the dose to one quarter

- Use forceps, manipulators etc.

(c) Use adequate shielding:
A radiation shield consists of layers of absorbing material which may be placed around a source to reduce the intensity. Placing shielding between yourself and a source of penetrating radiation will decrease your dose.

- For low energy beta and alpha emitters: (H-3, C-14, P-33 and S-35) shielding is not necessary. For high energy beta emitters (P-32 and Y-90), lucite is a commonly used type of shielding material . Thin layers of material can completely absorb alpha and beta particles; this thickness is called the range. When the beta particle collides with lead shielding, bremsstrahlung, a type of X-ray, is emitted and in some cases is sufficiently intense to produce an increased dose rate outside the shield. Care is needed to ensure that a beta particle shield does not increase the dose rate.

- For gamma or x-ray emitters (Cr-51, Tc-99m, Na-22, Na-24, I-125, I-131), lead is used when exposure rates are significant. The thickness of the shield to reduce the dose rate to one tenth is the tenth value thickness (TVT). The values depend on the energy of the radiation and the nature of the shield material.

• Alpha sources (simple thin plastic, paper etc.)
• Beta sources (use light material of thickness greater than range).
• Gamma sources (use lead or concrete to reduce exposure sufficiently)

(d) Provide proper interlocks, warning lights etc.

(e)  Perform swipe checks on sealed sources:
These checks must be performed on an annual basis to ensure the sources are not leaking.

The safe use of radioisotopes

There are 2 potential hazards associated with the use of sources of radiation

1. External radiation
2. Internal radiation

External doses of radiation can be minimised by the following practices as described above

• Minimise the time of exposure
• Increase the distance to the source
• Use adequate shielding

Sources of Internal Radiation

Internal radiation follows when radioisotopes are taken into the body and there are 3 main ways in which this can happen.

1. Ingestion via the mouth
2. Absorption via the skin, whether broken or entire.
3. Inhalation via the lungs.

Radiotoxicity associated with sources of internal radiation

Alpha particles and low energy beta particles represent little hazard with respect to external radiation because of their short maximum path lengths in air. These isotopes when taken into tissue may be very hazardous as the particles inflict biological damage over the whole length of their paths. Alpha particles are of particular concern because of their high linear energy transfer (LET) and the consequent intense ionisations produced by them. For this reason, alpha emitting isotopes are given the highest radiotoxicity classification and alpha radiation is assigned a high quality factor where absorbed dose is converted to dose equivalent.

 

General Guidelines for users

1. The minimum amount of radioisotope must be used consistent with the efficient performance of the task at hand (ALARA). For each new project or experimental involving radioisotopes a detailed protocol must be written and approved by the radiological protection officer. Information on the use of appropriate safety equipment and proper shielding should be provided. Advice should be sought from other users of radioisotopes to avoid common pitfalls.
2. Records of the reception of each shipment of radioisotope must be kept. A card/form must be filled out for each radioisotope shipment and on which details of each aliquot taken are entered. Each shipment of radioactivity must be swabbed to determine that no leakage or contamination has occured before or during transit. It is the responsiblity of the person who ordered the radioisotope to keep all records for that shipment.
3. Clearly marked containers should be used for radioactive compounds and kept in a definite area. Radioactive compounds kept for storage (e.g. in fridges or freezers) must be labelled with the name of the user, name and quantity of radioisotope is essential.
4. All radioactive tape and labels should be defaced prior to be being put in the specially designed waste containers and stored in appropriate period until disposed.

Personal Protection

1. Wear a dosimeter if appropriate (contact RPO).
2. Clean laboratory coats which buttons to the neck must be won at all time.
3. Disposable gloves should be worn when working with radioisotopes and checked regularly for contaimination. Used gloves should be disposed of in the appropriate waste container.
4. For iodination procedures it is essential that 2 pairs of gloves are worn at all times. Disposable aprons and overshoes must also be worn.

Contamination

Accidents or inadequate containment procedures lead to contamination. Significant contamination can lead to the intake of radioisotopes by the personnel involved. There are three types of contamination.

• Airbourne contamination: radioactive gases; aerosols and fine particles.
• Surface contamination.
• Contamination of skin and clothing.

It is essential that work areas are monitored for contamination before and after each procedure. Surfaces may be monitored by the use of suitable monitoring equipment. It is recommended that all surfaces are also swabbed to detect lower levels of contamination. The swabs before a procedure ensure that the prior user has not contaminated the area and ensures that all users maintain the same high standards. All results are recorded and retained for inspection by the radiological protection officer. Contaminated area are washed thoroughly and re-swabbed until counts are at background levels.

Waste procedures

The University of Galway has developed a method for the control of radioactive material. This method has two aspects, firstly knowing how much material is on-site and secondly making users responsible for waste. There are two pathways depending on the type of material. If the material has a short half-life (such as P-32, I-125, S-35) then it is stored until its activity has decayed away when it can be disposed as it is not radioactive. Long-lived isotopes (H-3 and C-14) are disposed of if they satisfy the conditions laid down in SI 125. Otherwise they are kept indefinitely. In all cases the material will only be disposed of if it is not a biohazard.

Short half-Life Material

When radioactive material comes into the college it must be signed for and its activity noted, person responsible indicated. The form (incoming isotope form) also prompts the user to swipe test the container to ensure that the material has not leaked in transit. As the material is used the remaining quantity is marked down on the form which is kept with the isotope. The material is stored in sacks without any trefoil signs (or with ones that have been defaced). Finally when it comes to disposing of the material another form (waste disposal form) is used which indicates who is responsible for the waste, what activity is involved, what isotope is involved etc.

Long Half-Life Material

Two procedures are used. Firstly if the material satisfies SI 125 - i.e. the activity and concentrations are lower than Table A of Schedule 5 - then it is disposed of in deep landfill or in a foul water drain if it is aqueous. If however it is of high activity it is then stored indefinitely.

Radiation Detection and Personnel Dosimetry

Ionising radiation is not detectable by our senses so some form of detector that converts radiation energy into a visible signal must be devised.

Monitoring of radiation exposure

Radiation exposure is the dose due to a radioactive source. As a general rule, the source will be external to the body and the consequence is an external dose. Radiation monitoring instruments respond by emitting a sound or by a signal on a visual display.

Monitoring of radioactive contamination

Radioactive contamination is unwanted radioactive material carried by air, water or deposited on a surface. It is responsible for the internal dose if a person inhales or ingests the material. Monitoring for contamination involves the measurement of the activity deposited on surfaces or dispersed in volumes of air or water. The discovery of activity on a surface is often a warning of contamination in the air.

The Survey Meter

The most versatile instrument is a radiation survey meter or monitor based on a Geiger-Muller counter. This device can be used for both radiation and contamination surveys.The principle of gas filled detectors, which includes geiger counters, proportional counters and ionisation chambers, is that the gas filled chamber collects ions produced by the radiation and measures the electric current produced. If a small beam of radiation is incident then a nearly steady but small current will be produced. If a small rate of particles is incident then very small pulses of current are produced which are only detectable if large internal amplification is present as in the proportional and geiger counters. The geiger counter usually consists of a gas-filled tube which gives an electrical signal when a gamma ray or beta particle is absorbed in gas. As the dose rate is increased, the tube emits the signal more rapidly. The signals may be made audible by a loudspeaker. The number of signals may be counted digitally. The rate of signals over a given period of time is the 'count rate'. In most cases this can be displayed on a meter. The wall of the counter tube may be very thin or it may have a thin area at the end (end window) to assist in the detection of beta particles.

 

Types of Radiation Detectors used in Radiation Protection

• Proportional chambers contain xenon gas and have sufficient amplification to ensure that the electrical signals produced in the gas are in proportional to the energy of the ionising particle. A spectrum can be deduced from an analysis of the pulses. The size of the pulses are proportional to the energy deposited in the chamber. Alpha particles generate bigger pulses than beta particles or X-rays and so can be distinguished electronically.
• Ionisation chambers also detect the ionisation in a gas-filled space between two electrodes but there is no amplification. The detector is not very sensitive but it has the advantage that the current is proportional to the dose rate without the need for energy compensation. The principle use is to measure the exposure from X or gamma ray sources where a very accurate and energy independent measurement is required. Ion chamber instruments measure X and gamma rays accurately from a lower energy of about 20 keV upwards; lower energy radiations are absorbed in the chamber walls.
• Scintillation counters respond to the light emitted when beta particles and gamma rays have been absorbed in the phosphor. There are many phosphor materials which include plastic and sodium iodide. They can absorb much of the energy from a gamma photon and all the energy of the beta particle so they can give an electrical signal or pulse which is a very good indication of the particle energy. A spectrum of particles energies can be obtained which will permit the identification of the radionuclide. Liquid scintillators are particularly good for measuring biological specimens containing low energy beta sources such as C-14 and H-3 as tracers.
• Solid-state conduction detectors use crystals of silicon or germanium in which electrical changes are caused by the absorption of radiation energy. The electrical changes are very small and so it is often necessary to cool the detector to a very low temperature in a bath of nitrogen liquid. It is possible to measure the spectrum very precisely with very little interference.
• Thermoluminescent detectors (TLDs) use crystalline solids such as lithium flouride. The crystals of the lithium flouride can absorb the energy from the radiation to produce excited metastable electron energy states. When heated this absorbed energy is given out in the form of light which can be measured with a photomultiplier. These detectors are widely used for personal dosimeters and for environmental monitoring.
• Personal Dosimeters. As well as monitoring the working environment for contamination it is also important to provide personnel with a detector which is worn during work hours and which gives the total equivalent dose received (sieverts). Such monitors are usually changed every two weeks or each month and doses received are recorded. Although any of the above may be carried by an individual and then act as a personal dosimeter. The film badge is one of the most common methods which employs the blackening of photographic film by ionising radiation. A piece of photographic film is inserted into a special holder which has different absorbing windows to distinguish different types of radiation. The ranges of readings varies between 200 uSv and 100 Sv.

Health effect of radiation exposure

Radioactive materials that decay spontaneously produce ionising radiation, which has sufficient energy to strip away electrons from atoms (creating two ions) or break some chemical bonds. Any living tissue in the human body can be damaged by ionising radiation. The body naturally tries to repair the damage, but sometimes the damage is too severe or widespread, or mistakes are made in the repair process. There are a number of characteristics specific to ionising radiation which differentiate it from chemical toxic reagents or other physical carcinogens.

1. Its ability to penetrate cells and to deposit energy within them in a random way, unaffected by the usual cellular barriers, which limit other chemical agents.
2. All cells in the body are thus susceptible to damage.
3. It is possible to measure accurately very low levels of exposure, doses several orders of magnitude below those that produce measurable biological effects in human cells.

In general, the amount and duration of radiation exposure affects the severity or type of health effect. There are two broad categories of health effects: stochastic and non-stochastic.

Stochastic Health Effects

Stochastic effects are associated with long-term, low-level (chronic) exposure to radiation. Increased levels of exposure make these health effects more likely to occur, but do not influence the type or severity of the effect. Cancer is considered the primary health effect from radiation exposure. Radiation's ability to break chemical bonds in atoms and in molecules makes it such a potent carcinogen. Radiation can also cause changes or 'mutations' in DNA. Sometimes the body fails to repair these mutations or even creates mutations during repair. The mutations can be teratogenic or genetic. Teratogenic mutations affect only the individual who was exposed. Genetic mutations are passed on to offspring.

Non-Stochastic Health Effects

Non-stochastic effects appear in cases of exposure to high levels of radiation, and become more severe as the exposure increases. Short-term, high-level exposure is referred to as 'acute' exposure. Unlike cancer, health effects from acute exposure to radiation usually appear quickly. Acute health effects include burns and radiation sickness. Radiation sickness is called 'radiation poisoning'. It can cause premature aging or even death. The symptons of radiation sickness include nausea, hair loss, weakness, skin burns.

There is no firm basis for setting a 'safe' level of exposure above background for stochastic effects. In fact many sources emit radiation that is well below natural background levels.