Handbook of preventive medicine.

  • Great Britain. Air Ministry.
Date:
1959
Catalogue details

Licence: Public Domain Mark

Credit: Handbook of preventive medicine. Source: Wellcome Collection.

    493/556 (page 423)
    This formula applies only to point sources. A source may be considered to be a point source when its dimensions are small compared with the distance involved. The ratio between the two should exceed 1 to 5. When a source is not a point source it is assumed to be a segment of a spherical shell with the centre at the point at which the dose rate (D) is required. Then, if D, is the dose rate of the surface of the source and ¢ the solid angle subtended by the source at the point of interest :— Pye ee 2 153. Point Source Approximations. The full calculation of dose rates is rather involved and in practice approximations are used :— (a) Gamma. The following approximation may be used for gamma emitting point sources :— Dose rate at 1 foot = 6 xX C xX E rads/hr. where C is the source strength in curies and Eis the total energy of the gamma radiation (MeV) released on each disintegration. The formula only holds for gamma rays between 0-3 and 3 MeV energy. The dose at 1 metre may be obtained by using 0:54 instead of 6 in the formula. The total energy is obtained from a radiosisotope data book or nuclide chart. For example, sodium 24 emits two gamma rays (both below 3 MeV), one of 1-37 and the other of 2-75 MeV, with each disintegration. The total energy is therefore 4-12. Sometimes only a certain proportion of disintegrations are accompanied by the release of gamma rays. For example, potassium 42 releases gamma rays of 1-52 MeV in 18% of its disintegrations. The total energy, therefore, to be inserted in the formula is:— 18 100 pale 52 (b) Beta. The dose rate per hour from a beta source of C curies at 1 foot is given by the following formula if self absorption and air absorption are neglected (neglecting these over estimates the dose and is therefore safe) :— Dose rate at 1 foot = (approx.) 300 x C rads/hour Fission Products 154. Radiac Calculator No. 1 In the area of intense fall out from a nuclear weapon explosion the gamma dose rate should be measured about a metre above the ground. If the time that has elapsed since the explosion occurred is known, the Radiac Calculator No. 1 will make it possible to determine the dose rate in the same spot at any future or past time, the dose that will be received by staying in the area between two stated times and how long it is possible to remain before any specified dose is received. The instructions for use are given on the reverse side of the calculator. 155. Fission Product Decay. The radionuclides formed in the fission chain reaction which occurs when a nuclear weapon is detonated decay by beta emission, accompanied in the majority of cases by gamma radiation. The gamma activity in roentgens per hour at t hours after the explosion is given by the use of the formula :— 423