(31 p.780) Ionizing Radiation Umm Al-Qura University جا معة أم القرى Faculty of Applied Science كـلية العلوم التطبيقية General Physics 104 (For medical students) فيزياء طبية 104 (31 p.780) Ionizing Radiation Hamid NEBDI, Ph. D A.Professor. Department of Physics Faculty of Applied Science.

Introduction The radioactive decay of nuclei produces several kinds of ionizing radiation (positive ions, electrons and positrons, photons, neutrons, …) with energies that are typically several MeV per particle or quantum. When this radiation passes through matter, it leaves a trail of ionized atoms along its path. A small amount of ionization can seriously disrupt a sensitive system such as a living cell or a transistor. The term “ionizing radiation” includes both nuclear radiation and atomic X rays.

1. Radiation Units Four types of radiation measurements are used in various applications: 1- Source activity 2- Exposure 3- Absorbed dose 4- Biologically equivalent dose

1.1 Source activity (1) The source activity A is the disintegration rate of a radioactive material or the rate of decrease in the number of radioactive nuclei present. It is measured in curies, where the curie is defined by the exact relationship: 1 curie= 1 Ci = 3.7 x 1010 disintegration per second One gram of radium has approximately 3.7 x 1010 disintegrations per second, so that 1 Ci of radium has a mass of about 1 g. A curie is a fairly large unit, a 1 Ci radioactive source requires shielding and careful handling. Radioactive sources used in an introductory physics laboratory experiments have an activity measured in microcuries. The S.I. source activity unit is the becquerel (Bq), which is one disintegration per second.

1.1 Source activity (2) The activity of n moles of a sample is then : The activity A of a sample is related to its half-life, T: A = -  N/ t = N = 0.693 N/T If there are n moles in the sample, then the number of atoms is: N = n NA where Avogadro’s number NA = 6.02 x 1023 is the number of particles in a mole. The activity of n moles of a sample is then : A = 0.693 n NA/T

Example 31.2: What is the mass of a 1000 Ci cobalt source? 60Co beta decays with a half-life of 5.27 years = 1.66x108s into 60Ni, which then promptly emits two gamma rays. These gamma rays are widely used in treating cancer. What is the mass of a 1000 Ci cobalt source?

Solution 31.2: From the following equation: A = 0.693 n NA/T the number of moles n is : n = A T /0.693 NA so n = 1000 (3.7x1010 s-1)(1.66x108 s)/0.693(6.02 x 1o23 mole-1) n=0.0147 mole Since a mole of 60Co has a mass of 60 g, the mass of the sample is: m =(0.0147 mole)(60 g mole-1)=0.882 g

1.2 Exposure Exposure indicates the amount of radiation reaching a material. Exposure depends on the characteristics of the beam alone. Exposure is defined only for X rays and gamma rays with energies up to 3 MeV and not for other forms of radiation. It is defined as the amount of ionization produced in a unit mass of dry air at standard temperature and pressure (STP), 1 atmosphere and 00 C. The conventional unit is 1 roentgen =1R=2.58x10-4 C/kg. 1 roentgen of X rays will produce 2.58x10-4 C of positive ions in a kilogram of air at STP, and an equal amount of negative ions.

1.3 Absorbed Dose Absorbed dose indicates the energy absorbed in the material from the beam. The absorbed dose depends on the properties of the material and the beam. The absorbed dose is the energy imparted by ionizing radiation to a unit mass of absorbing tissue. It is measured in rads, where 1 rad = 0.01 joule per kilogram A 1 roentgen exposure to X rays or gamma rays produces a soft tissue absorbed dose of approximately 1 rad. The official S.I. unit is the gray (Gy), which is 1 joule per kilogram or 100 rads. Unlike exposure, the absorbed dose is used with all kinds of ionizing radiation, not just X rays and gamma rays with energies below 3 MeV.

1.4 Biologically equivalent dose (1) The absorbed dose refers to a physical effect: the transfert of energy to a material. The effects of radiation on biological systems also depend on the type of radiation and its energy. The quality factor (QF) of a particular radiation is defined by comparing its effect to those of a standard kind of radiation, which is usually taken to be 200 keV X rays. For example, fast neutrons (with energies above 0.1 MeV) have a QF of about 10 for causing cataracts. Hence the absorbed dose (in rads) of 200 keV X rays needed to produce cataracts is 10 times the dose required for neutrons. The QF varies with the radiation type and energy, with the animal species, and the biological effect under consideration (Table 31.1). The rem and the millirem = 10-3 rem are the units used in discussions of biological effects.

1.4 Biologically equivalent dose (2) Typical QF Radiation 0.7 1.17 and 1.33 MeV)) 60Co γ rays 0.6 γ rays 4 MeV 1.0 β particles 2 Protons (1 to 10 MeV) 2-10 Neutrons 10-20 α particles Table 31.1: Typical QF values. By definition, the QF is exactly 1 for 200 keV X rays.

1.4 Biologically equivalent dose (3) In any situation the biologically equivalent dose (in rems) is the physical absorbed dose (in rads) time the QF. One rem of any kind of radiation produces the same biological effect, namely, the effect of one rad of 200 keV X rays. In S.I. units, the biologically equivalent dose in sieverts (Sv) equals the dose in grays times the QF. Definition Unit 3.7 x 1010 disintegration per second disintegration per second curie (Ci) becquerel (Bq) Source activity 2.58x10-4 C kg-1 in dry air at STP R))roentgen Exposure (X and γ rays) 0.01 J kg-1 1 J kg-1 rad gray (Gy) Absorbed dose QF x (dose in rads) QF x (dose in grays) rem sievert (Sv) Biologically equivalent dose Table 31.2: Radiation units

Example 31.4: Which have a QF of 0.7. Find: 60Co A cancer is irradiated with 1000 rads of gamma rays, Which have a QF of 0.7. Find: a- The exposure in roentgen b-The biologically equivalent dose in rems

Solution 31.4: a- For gamma rays, the soft tissue dose in rads is approximately equal to the exposure in roentgens, so the exposure is approximately 1000 R. b- The biologically equivalent dose is the product of the QF and the dose in rads: o.7 x 1000 rads = 700 rems

Example 31.5: a- How many decays occur per hour? A laboratory experiment in a physics class uses a 10 microcurie source. Each decay emits a 0.66 MeV gamma rays. a- How many decays occur per hour? b- A 60 kg student standing nearby absorbs 10 percent of gamma rays. What is her absorbed dose in rads in 1 hour? c- The QF is 0.8. Find her biologically equivalent dose in rems. 137Cs

Solution 31.5: a- Since 1 curie products 3.7x1010 decays per second, the number of decays in an hour for a 10 microcurie source is: N= (10x10-6)(3.7x1010 s-1)(3600s)=1.33x109 b- Each decay produces 0.66 MeV, of which 10 percent is absorbed. Thus the total energy absorbed is: E=(0.1)(1.33x109)(0.66 MeV)(1.6x10-13 J MeV-1) = 1.41x10-5 J Dividing by her mass, the absorbed dose in rads is: E/m = (1.41x10-5 J)(1rad)/(60 kg)(0.01 J kg-1) c- Multiplying the dose in rads by the quality factor gives the biologically equivalent dose in rems: (E/m)(QF)= (2.34x10-5 rad)(0.8)=1.87x10-5 rem

2. Harmful effects of radiation (1) -When radiation passes through living cells, it can alter or damage the structure of important molecules. - This may lead to the malfunctioning or death of the cells and ultimately to the death of the organism. Usually, immature cells (fetuses), or the cells that are growing (infants) or dividing are the most radiosensitive. Often cancer cells are rapidly growing, in such cases, they are highly vulnerable to radiation. For example, in one study it was found that children whose mothers received pelvic X rays while pregnant had a 30 to 40 % increase in the incidence of cancer.

2. Harmful effects of radiation (2) -The limited knowledge of the immediate effects on humans of large radiation doses comes from studies of victims of atomic bomb explosions and of occasional accidents. - For : *whole body doses < 25 rems  No observable effect *doses >100 rems  Damage to the blood-forming tissues becomes evident *doses >800 rems  Severe gastrointestinal disorders Death usually follows in a period of days or weeks if the dose is much more than about 500 rems. Sublethal, short-term doses and doses acquired gradually over a long period may lead to cancer after a latent period of many years during which no ill effects are discernable. The chance of dying of cancer is doubled by a dose somewhere between 100 and 500 rems.

2. Harmful effects of radiation (3) - Studies of low-level doses are difficult and inconclusive, because of the much larger incidence of cancer from other causes. Consequently, it is possible that the damage resulting from doses below some threshold is repaired, so that no increase in the cancer rate occurs. For many years, most experts favored the more conservative linear hypothesis, which assumes that the effects of radiation in causing cancer are proportional to the dose at all levels. In 1980, they found that the linear hypothesis probably overestimates the effects of low-level exposures.

2. Harmful effects of radiation (4): Genetic effects - Most genetics mutations are harmful. Any increase in the mutation rate means more prenatal deaths and more people born with serious defects. Mutations can be increased above the normal rate by: *elevated temperatures * some chemicals * ionizing radiation The mutations rate caused by radiation is proportional to the absorbed dose, no matter now small, and that there is no threshold or repair mechanism. The dose that will double the mutation rate is probably between 25 and 150 rems. For more information about the maximum permissible doses see Table 31.4