dose rate calculations
Dose rate Calculations
No student is permitted to proceed to their experiment until they have verified that the dose rate they could receive is well within the limits laid down and have done the calculations enclosed with the notes on the experiment.
- What is the dose rate 2m from a Co60 source of strength 1mCi?
- A cobalt 60 source has a dose rate of 40 mSv/hr at 1 m. What thickness of lead is required to reduce this dose to 2.5 µSv/hr?
Background Annual Exposures
In areas of normal background the average annual effective dose equivalent rom radiation of natural origin, from cosmic radiation, terrestrial gamma-radiation, internal radioactivity and inhaled radon is 2.0 mSv. In addition there is a population average dose of 0.25 mSv from medical procedures and 0.03 mSv from the sum of all other sources. The annual total is therefore 2.3 mSv.
Maximum Permitted Dose Rates
The International Commision on Radiological Protection has laid down a maximum permissible additional dose for ‘members of the public’ of 5 mSv in a year (students working in the Level 3 Nuclear Laboratory are classified as ‘members of the public’). The maximum dose-rate at the position of working is 2.5 micro-Sv hr-1. This upper limit must be absolutely adhered to; and efforts should be made to keep well below the limit. A radiation monitoring instrument is available for measuring dose-rates at the seating positions of the students. No student is permitted to proceed to carry out their experiment until they have verified that the dose-rate they could receive is well within the limits laid down.
Radioactive Units
Unfortunately there are many definitions for the radioactivity of a source. Although some consensus has been made in defining SI units there are still many texts using antiquated or non-standard units. The units used are based upon either
- the numbers of quanta of a particular radiation, or
- the effects produced by that radiation such as ionization or biological damage
Below is a summary of the types of units that you may come across and conversion factors for them.
- The Becquerel. Defined as 1 disintegration per second. This is the SI unit for measuring radioactivity.
- The Curie. Defined as 3.7 x 1010 disintergrations per second.
This is the activity of 1 gram of radium in equilibrium with its decay products. - The Rutherford. 1 Rd = 106Bq.
- The Roentgen. This is defined as the amount of gamma radiation that produces 1 cm3 of air ionization equal to 1 esu.
1 esu = 3.3 x 10-10 coulombs = 2 x 109 ion pairs/cm3 of air. Equivalent to 2.58 x 10-4C/kg air (0.0087 J/kg of air).
1R is approximately 10-2 Sv.
- The Gray. 1Gy = 1 J/kg. This is the SI unit for absorbed dose of ionizing radiation.
- The Rad. 1 rad = 10-2Gy (= 10-2 J/kg). This is defined as the amount of radiation that deposits 100 ergs (10-5 J) in each gram of tisue it traverses. Two different types of radiation may, however, produce different degrees of biological damage even though they are both rated as 1 rad.
- The rep (Roentgen equivalent physical). It has been found that 1R of gamma radiation dissipates an energy of 93 erg/gm (93 J/kg) of average tissue. Hence the rep is defined as that quantity of gamma-radiation which suffers an absorption of in tissue of 93 erg/gm.
- The Sievert. 1 Sv = 1 J/kg. The Sievert is equal to the absorbed dose in tissue (Gy) multiplied by the ‘quality factor’ for the particular type of ionising radiation. The quality factor is a dimensionless number representing the relative effect produced by the same absorbed doses of different types of radiation. In older terminology the quality factor was referred to as the Relative Biological Effectiveness (see table below)
- The rem (roentgen equivalent man). This is defined as the amount of radiation which when absorbed by a person, will produce the same biological effects as the absorption of 1 roentgen of x-ray or gamma-ray radiation. To define the rem quantitively, a relative biological effectiveness (RBE) has been established and (number of rem) = (number of rad) x RBE. The following table gives RBE for the usual types of radiation.
RELATIVE BIOLOGICAL EFFECTIVENESS
| Type of radiation | rad | x RBE | = rem |
| x-rays and gamma-rays | 1 | 1 | 1 |
| Beta radiation | 1 | 1 | 1 |
| Protons | 0.1 | 10 | 1 |
| Alpha particles | 0.05 | 20 | 1 |
| Alpha particles | 0.1 | 10 | 1 |
| Slow Neutrons | 0.3 | 3 | 1 |