WHAT IS IONIZING RADIATION

Transcription

1 WHAT IS IONIZING RADIATION Margarita Saraví National Atomic Energy Commission - Argentina Workshop on Ionizing Radiation SIM Buenos Aires 10 November 2011

2 What is ionizing radiation?

3 What is ionizing radiation?

4 Types of ionizing radiation The three main types of ionizing radiation are alpha particles, beta particles, and gamma rays. alpha particles, beta particles gamma rays

5 Types of ionizing radiation Penetration Beta decay is a radioactive process in which an electron is emitted from the nucleus of a radioactive atom, along with an unusual particle called an antineutrino. The neutrino is an almost massless particle that carries away some of the energy from the decay process. Because this electron is from the nucleus of the atom, it is called a beta particle to distinguish it from the electrons which orbit the atom.

6 Types of ionizing radiation Penetration

7 Types of ionizing radiation Penetration Alpha decay is a radioactive process in which a particle with two neutrons and two protons is ejected from the nucleus of a radioactive atom. The particle is identical to the nucleus of a helium atom. Alpha decay only occurs in very heavy elements such as uranium, thorium and radium. The nuclei of these atoms are very neutron rich (i.e. have a lot more neutrons in their nucleus than they do protons) which makes emission of the alpha particle possible.

8 Types of ionizing radiation

9 Types of ionizing radiation Gamma rays are electromagnetic radiation given off by an atom as a means of releasing excess energy. They are bundles (quanta) of energy that have no charge or mass and can travel long distances through air (up to several hundred meters), body tissue, and other materials. A gamma ray can pass through a body without hitting anything, or it may hit an atom and give that atom all or part of its energy. This normally knocks an electron out of the atom, ionizing it. This electron then uses the energy it receives from the gamma ray to create additional ions by knocking electrons out of other atoms. Because a gamma ray is pure energy, it no longer exists once it loses all its energy. The capability of a gamma ray to do damage is a function of its energy, where the distance between ionizing events is large on the scale of the nucleus of a cell.

10 Types of ionizing radiation Additional forms of ionizing radiation beyond the three types shown in the figure above include neutrons, protons, neutrinos, muons, pions, heavy charged particles, X-rays and others.

11 Types of ionizing radiation Neutrons, on the other hand, having zero electrical charge, do not interact electromagnetically with electrons, and so they cannot directly cause ionization by this mechanism. However, fast neutrons will interact with the protons in hydrogen (in the manner of a billiard ball hitting another, head on, sending it away with all of the first ball's energy of motion), and this mechanism produces proton radiation (fast protons). These protons are ionizing because they are charged, and interact with the electrons in matter. A neutron can also interact with other atomic nuclei, depending on the nucleus and the neutron's velocity; these reactions happen with fast neutrons and slow neutrons, depending on the situation. Neutron interactions in this manner often produce radioactive nuclei, which produce ionizing radiation when they decay, then they can produce chain reactions in the mass that is decaying, sometimes causing a larger effect of ionization.

12 INTERACTION OF IONIZING RADIATION WITH MATTER Interaction of ionizing radiation with matter

13 Nuclear reactions: fission The spliting of a nucleus with a large mass into two nuclei with smaller masses.

14 Nuclear reactions: Fusion Fusion of deuterium with tritium creating helium-4, freeing a neutron, and releasing MeV of energy, as an appropriate amount of mass changing forms to appear as the kinetic energy of the products, in agreement with kinetic E = Δmc2, where Δm is the change in rest mass of particles

15 Isotopes and Activity Isotopes Atoms in their normal state are electrically neutral. They have a nuclei composed by neutrons and protons and the electrons are in orbits around the nuclei. m p = x kg m n = x kg m e = x kg Atoms with the same number of protons and different number of neutrons are called ISOTOPES. An isotope may be defined as one or two or more forms of the same element having the same atomic number (Z), differing mass numbers (A), and the same chemical properties.

16 Activity The activity of a radioisotope is the number of radioactive disintegrations per a unit of time. The SI unit for measuring the rate of nuclear transformations is the becquerel (Bq): 1 radioactive disintegration per second. The old unit for this is the curie (Ci): 3.7 x radioactive disintegrations per second.

17 Activity N = -. t N = N 0. e -.t.n =.N 0. e -.t A = A 0. e -.t Half life T 1/2 = ln2/

18 Dose and Source Dose The amount of energy of any type of ionizing radiation imparted to (or absorbed by) a media The international (SI) unit of measure for absorbed dose is the gray (Gy), which is defined as 1 joule of energy deposited in 1 kilogram of mass.

19 Dose and Sources Equivalent Dose To look at biological effects, we must know (estimate) how much energy is deposited per unit mass of the part (or whole) of our body with which the radiation is interacting. Equivalent dose the biological effect depends not only on the amount of the absorbed dose but also on the intensity of ionisation in living cells caused by different type of radiations. Neutron, proton and alpha radiation can cause 5 to 20 times more harm than the same amount of the absorbed dose of beta or gamma radiation. The unit of equivalent dose is the sievert (Sv). This unit was developed to allow for the consistent reporting of hazards associated with the various types and energies of radiation on the human body. The Sv is the product of the absorbed dose (the amount of energy imparted to tissue by the radiation), and factors for the relative biological effectiveness (RBE) of the radiation.

20 Sources of Radiation Exposure Radiation is permanently present throughout the environment, in the air, water, food, soil and in all living organisms. Large proportion of the average annual radiation dose received by people results from natural environmental sources. Each member of the world population is exposed, on average, to 2.4 msv/yr of ionizing radiation from natural sources. In some areas (in different countries of the world) the natural radiation dose may be 5 to 10-times higher to large number of people.

21 Where Does Ionizing Radiation Come From?

22 How is Ionizing Radiation Used?

23 What s in the Environment? Exposure to background radiation and naturally occurring radioactive materials results in an annual dose of about 2.5mSv/yr. Of this total, about one-third is due to external ionizing radiation, the main contributors being cosmic rays and terrestrial gamma rays (0, 29 msv/yr each), and radionuclides within the body (0,40 msv/yr). Cosmic rays are produced when subatomic particles originating outside the solar system interact with particles in the upper atmosphere to produce gamma rays, neutrons and leptons that can reach and penetrate the earth s surface. It is these secondary particles and rays that produce the dose from cosmic radiation.

24 What s in the Environment? About two-thirds of the background radiation dose (of 2.5 msv/yr) is due to intake of radionuclides into the body. The largest contributor is inhalation of radon-220 and radon- 222 gases and their short-lived radioactive decay products, PO-218 and Po-214 which are charged particles that readily attach to airborne dust particles. Ingestion of food and water containing naturally occurring radionuclides accounts for only a few cents msv/yr.

25 What Are the Primary Health Effects? High doses of ionizing radiation can lead to effects such as skin burns, hair loss, birth defects, illness, cancer, and death, depending on the dose and the period of time over which it is received. Acute doses (such as from a serious accident involving nuclear materials) can result in damage to the blood-forming organs, gastrointestinal tract, and central nervous system. Very high doses, e.g., on the order of 500 rem (or 500,000 mrem) or more, can cause death in many (but not all) individuals, depending on the degree of medical intervention. The main health concern associated with radiation exposure is the induction of various cancers. Additional effects may include genetic mutations (although none have been observed in humans) and teratogenic effects such as mental retardation.

26 What Happens to it in the Body? Radioactive materials can enter the body by inhalation, ingestion, Gamma radiation external to the body can penetrate the skin and produce a dose in various tissues.

27 What Is the Risk? While some national and international associations have developed lifetime cancer mortality risk coefficients for nearly all radionuclides, they have not developed a risk coefficient for ionizing radiation as a general category. A nominal mortality value of incremental cancer risk per 0.01 msv has been identified for low-let radiation delivered at a low dose and dose rate.

28 What s in the Environment? Naturally occurring radioactive elements such as uranium, thorium and radium are present in soil, rock, water, and all other environmental media, and certain of these radionuclides (and their radioactive decay products) give off gammarays as they undergo radioactive decay. The principal contributor to the dose from radionuclides in the body is potassium-40,which decays by emitting an energetic beta particle and gamma rays.

29 How is Ionizing Radiation Detected and Measured? NaI(Tl) scintillation detectors

30 How is Ionizing Radiation Detected and Measured? Germanium Detectors

31 How is Ionizing Radiation Detected and Measured? Dosemeters Measures are based on the absorption of energy in a media. Results: determination of absorbed dose

32 Thank you!