Chapter 1 Introduction of Radiation and Radioactivity

Transcription

1 Chapter 1 Introduction of Radiation and Radioactivity 1

2 1.1 Introduction The 19th- century scientists knew little about what went inside an atom and at the end of the century there were found the new ideas about the structure of atom and radiation emitted by it. The radioactivity and the existence of the nucleus were discovered as the consequence of apparently incomprehensible experimental observations. Becquerel a spent long time on the study of various fluorescent materials and fortunately he took the double salt potassium uranly sulphate and placed it on the photographic plate wrapped in black paper and exposed to the sunlight; when the photographic plate developed, it was found to be darkened indicating that the uranium salt emitted radiation which could penetrate the black paper he further concluded that this invisible radiation like x rays. He showed subsequently that the radiation which was affecting the photographic plate depended neither on the particular chemical compound used nor on its temperature or other external circumstances. Then Pierre and Marie Curie isolated from uranium ore (pitchblende) two elements either to unknown (at that time)-polonium and radium which were far more active than uranium. It was named polonium, in honor of his native place Poland and it was found that the radium is several million times active than the uranium. Rutherford demonstrated that radiation from the radioactive material had the composition of two rays he called α rays and β rays. Villard discovered a third type radiation, which had more penetrating power and named as γ rays. The spontaneous emission of α, β, and γ rays are the atomic phenomenon which was proved by Becquerel. The radiation was a property of the atoms themselves and that it was capable of ionizing gasses. This radiation carried with a great deal of energy, since in spite of the very small quantities of emitting matter; it produced an observable heating effect in anything that absorbed it. The physicist Becquerel detected the existence of radiation from the uranium-bearing rocks and he had encountered radioactivity; all unstable nuclei, as well as an nuclei in the excited state undergo spontaneous transformations leading to a change in their composition or internal energy of the nucleus. Tayal D. C. (2008). 2

3 We are exposed to radiation from the sun, outer space and also from the naturally occurring radioactive materials present on the earth, the house we live in, the buildings where we work, the food & drink. There are radioactive aerosols and gasses in the air, we breathe and even our own bodies contain naturally occurring radioactive elements. All materials on the Earth are constantly exposed to the radiation that may be non-ionizing radiation or ionizing radiation according to their energy that may be manmade or naturally produced. Nature provides essential shielding to the atmosphere like earth dipole magnetic field protect us from external solar and galactic cosmic rays but the gamma radiation which is used in the medical for radiation theory, chemotherapy etc. to treats the cancer by targeting the tumor or cancerous area and mutating the proliferating cell so they can no longer reproduce and therefore die unfortunately; healthy cells within the patient s body can be destroyed by the gamma radiation, This is why there is a need for tissue compensators as well as radiation protection in radiation treatments for this we know the essential shielding and shielding materials for the radiation protection. To understand the biological effect of radiation, we must understand the difference between ionizing radiation and non-ionizing radiation; Ionizing radiation has enough energy to eject electrons which cause the produced ions, so the ionizing radiation produced a number of physiological effects, such as the risk of cancer. The nuclear radiation is the ionizing radiation because of their high energy but the nuclear radiation is used in radiation therapy and medical imaging to diagnose the cancerous cells or the abnormal tissues are good side of it; for these treatments the different radioisotopes are used, therefore the researcher are in the development of medical isotopes, their production, and processing. In the field of preclinical testing, clinical evaluation of the agents and evaluation of new pharmaceuticals for the application of in nuclear medicine, oncology and interventional cardiology have been focused as number one priority. Gupta T. K. (2013), National Research Council and Forshier S. (2001). The transmission measurement of the intensity of gamma rays through the different shielding materials (attenuators) are the useful to characterizes the gamma rays in term of the linear attenuation coefficient, half value thickness and the mass attenuation coefficient of attenuator for the corresponding energy of gamma rays; which represent a subject of great interest and importance to the scientist due to its 3

4 utility in solving various problems related to radiation physics and radiation dosimetry. The, X 1/2, m, Tot and el are the quantities useful in determining the penetration, penetration depth, the probability of interaction of gamma rays photons in the matter; These quantities can be evaluated theoretically and experimentally when a beam of gamma rays fall on a material. Incident photons may be absorbed, scattered and are transmitted through the material by interaction with atoms of the material. The linear attenuation coefficient and mass attenuation coefficient of the elements, mixtures and materials are widely used in space physics, dosimetry, radiation physics and many other applications in radiation studies. Ronald L. K. (1995), National Council on radiation protection and measurements (1997) and Syed N. A. (2007). Early measurements of the absorption coefficient of the gamma rays were hampered by the difficulty of producing sources of monochromatic radiation. This difficulty has recently been overcome by the use of radioactive isotopes and several workers have made an accurate measurement in this way. Most of the measured values of absorption coefficients agree with theoretical values within the limit of experimental errors. With the increasing use of radio-isotopes in various fields the studies of the absorption of gamma radiation in material become an important subject, for these studies the setup of geometry plays an important role in the measurement of linear, mass attenuation coefficient and other parameters. There are significant numbers of reports in the literature on the theoretical and some of experimental determination of these parameters of various elements, compounds and mixtures in the different energy range reported in Berger M. J. and Hubbell J. H. (1987), Khan F. M. (2003), Saloman E. B. et al (1988) and Tupe V. A. (2012). 1.2 Radioactive Nuclei and Disintegration The excited nucleus immediately decays into two or more daughter nuclei by emitting radiation so radioactive nuclei or unstable nuclei are transformed into a stable nuclide called the daughter by emitting the radiation. If the daughter product is also radioactive, process continuous until a stable product is reach. Radioactivity is a random process, it is not predicted to know exactly when an unstable nucleus will decay and can only specify a probability per unit time that it will do so. This is normally described by half-life, which the time taken for half nucleus in a sample to 4

5 decay. The radioactive decays of naturally occurring minerals contemning uranium and thorium are in large part responsible for the study of nuclear physics. In nuclear fission or the breakdown of an atomic nucleus neutron or protons are produced. The neutrons produced in this reaction can cause radiation protection problems if they are not accounted for. This type of reaction does not occur in this experiment because the energy levels used are much lower than the threshold for this type of reaction. All naturally occurring and artificially produce nuclei are either emit α, β and γ or a combination of α, β and γ radiation so the radioactive nuclei and its derivative may also emit such radiation. Activity of a radioisotope measures the disintegration rate, which is not same as the emission rate of radiation produced in its decay. The activity of a radioisotope source is defined as its rate at which the constituent atoms disintegrate per unit time and is given by the fundamental law of radioactive decay dn dt N [1.1] Where, dn is the number of radioactive nuclei decay in time interval dt, N is the number of radioactive nuclei present at the time of decay and λ is the decay constant, the negative sign indicates that the number of atoms is decreasing with time. The activity of a sample depends on the number of atoms in the samples that is the mass of the sample and upon type of an atom. The most way to measure the activity of material is to see how much disintegration per second it is going through, therefore the unit of activity is defined in term of disintegration per second. The S.I. unit of activity is Bq (Becquerel) and is defined as the radioactive substance gives one designation per second. The traditional unit of activity is Ci (curie) (in C.G.S. unit) and one curie is defined as the radioactive substance which gives 3.7 x disintegrations per second; interconversion factor between curie and becquerel is 1 Bq = Ci or 1 Ci=3.7x10 10 Bq 5

6 Rd (rutherford) is also used as unit of activity and defined as the radioactive substance which gives10 6 disintegration per second; the interconversion factor of rutherford and becquerel is 1 Rd = 10 6 Bq. Dosimetry Units: The amount of energy ( E) imparted by the ionizing radiation to the matter in a volume of element and m is the mass of the matter in the volume element is defined as absorbed dose. The special unit of absorbed dose is the rad (Radiation Absorbed Dose) and is defined as the absorption of 100 erg per g of the tissue i.e. 1 rad = 10 2 erg/g or 1 rad = 10-5 J/g or 1 rad = 6.24 x ev/g. The S.I. unit of absorbed dose is gray (Gy) and interconversion factor of gray and rad is 1 Gy = 10 2 rad. 1.3 Nuclear Instability and Isotopes Radiation is found everywhere and we are always exposed to the radiation that comes from the sky, the ground and air we breathe; the major components of 238 background dose are from the uranium decay chain. The radio nuclei ofu, U and 232 Th these are the main naturally occurring and long-lived half-life radio nuclides. These nuclides are the start of a long chain of radioactive isotopes ending with stable products. One element that is Radon is an odorless and colorless gas that decays to a series of radioactive metals so the most of the natural dose comes from the inhalation of Radon daughters and dose coming from the earth is primarily due to gammas emitted during these chains. An unstable nuclei try to get stable by decay alpha, beta, and gamma rays, but their derivative may be radioactive and it again emits such radiation until it s become a stable; according to the Rutherford unstable nuclei and is derivative become stable by emitting the alpha, beta, and gamma rays; during the decay, the nuclei changes depending upon the decay as follows 235 6

7 1.3.1 Alpha Decay Many naturally occurring heavy nuclei such as Uranium (U), Thorium (Th) and Radium (Ra) are the neutrons rich that make the decay through the emission of an alpha particle. In which the mass number of the atom decreased by four and atomic number decreased because loss of four nucleons in term of alpha particle from the parent nucleus; that four nucleons containing two neutrons and two protons so the atomic number decreased by two, as a result, the loss of two protons atom become new element (daughter nuclei). Alpha particles consist of two neutrons and two protons which are bound together and identical to a helium nucleus. The daughter nuclei may or may not be stable but lie closer to the region of stable nuclei and α particle has a very stable, tightly bound structure. The isotopes emitting alpha particles causes more damage to human body cell compared to the other ionizing radiation or particle due to their relative biological effectiveness. The neutrons rich heavy nuclei are energetically unstable against the spontaneous emission of an alpha particle (or He 4 nucleus). The decay process is written as Z A X [1.2] A 4 4 Z 2Y 2 Where, X and Y are the initial and final nuclear species, an example of alpha decay of uranium is shown in figure 1.1 Fig. 1.1 Alpha Decay of Uranium The alpha particles are relatively large and positively charged; therefore they do not penetrate through the matter very well so the thin piece of paper can stop alpha particle. 7

8 1.3.2 Beta Decay Unstable nuclei which are described as either neutron-rich or proton-rich decay towards the stability into other isobaric nuclei by negative or positive β particle emission or by the capture of an atomic electron; Beta (β) decay is the process of spontaneous emission of the high energetic electron that is emitted by the nucleus. In this decay process the mass number of nucleus remain unchanged but the atomic number changes because of the conservation charge. The change in atomic number is accomplished by the emission of electron, emission of a positron or by the capture of an orbital electron. So the three modes of beta decay as, a) β - Decay (electron emission) b) β + Decay (positron emission) and c) Electron captures (EC). The radioactive elements emitted β - (electron) the daughter elements has the same mass number as parent element, but the atomic number increases by one, while the element emitted β + (positron), the daughter nucleus has same mass number but atomic number deceased by one. Positron (β + ) is identical to an electron but having positive charged. When the ratio of neutrons to protons is low then the nucleus capture orbital electron and daughter nucleus has the same mass and atomic number decreased by one same as in the case of positron emission. The process is written as A A Z X Z 1 Y Or A A Z X Z 1 Y [1.3] Where, X and Y denote the initial and final nuclear species. Example of beta decay is shown in figure 1.2 Fig. 1.2 Beta Decay of Thorium 8

9 When the parent nucleus emitted the beta particle and become the new element i.e. daughter nucleus may in an excited state and de-excited by the emission of a gamma ray. The beta particle is light mass than the alpha particle. Only significant ionizing radiation produced by beta decay and has more energetic than the alpha particle so their penetration power in the matter is more than alpha particle but a small aluminum plate stop beta particles Murugeshan and Kiruthiga (2008) and Paic G. (1988) Gamma Decay The excited nucleus may lose energy in a transition to a lower level in the form of γ- ray photon or nucleus may be de-excited by ejecting an electron from one of the atomic orbits in the competing process called as internal conversion. If the transition energy sufficiently high, the third type of electromagnetic decay is possible called as internal pair formation, for this the transition energy is about MeV. Gamma (γ) decay of life time is very short compared with α and β decay life time. In gamma decay, the final nucleus is the same as the initial one with the same mass number (A), atomic number (Z) and number of neutrons but in a lower energy state. The energy of spontaneously emitted γ photons is the energy difference between transition levels of the excited nucleus. The ionization power of emitted γ photon is less compare to that of α and β rays but they are more penetrating than α and β rays and they are not affected by the electric and magnetic fields. Excited nucleus emits single γ photon if there is single radioactive transition to the ground state, and emits several γ photons if the cascade transition to the ground state, Here some of the examples of the radioactive isotopes with their half life and stable end product is shown in table 1.1 by Tayal D. C. (2008) and Profio A. E. (1978). Radioactive series Parent Half life Stable end product Thorium Neptunium Uranium Actinium Th 1.39 x 1010 Pb Np 2.25 x 106 Bi U 4.51 x 109 Pb U 7.07 x 108 Pb 82 Table 1.1 Example of radioactive series 9

10 1.4 Radiation Hazards and Safety We received some natural or background radiation exposure each day from the sun, radioactive elements present in the soil and rocks, household appliances and sometime medical. These levels of radiation are normal but the radiation from the reactors; material released through accidents or through the testing of nuclear devices spreads into the atmosphere is higher than that is other sources. If the Radioactive materials are handled improperly or the radiation accidentally released into the environments is dangers to the body because of its harmful effect. The persons are longer from the exposed radiation has a low risk of its harmful effect but persons are closer to the radiation has the greater risk. The International Commission on Radiological Protection has recommended the certain maximum permissible exposure to the radiation that no appreciable injury to the persons. The radiation exposure can be reduced to the safe limits by the following process By decreasing the time spent near a source, by allowing the source to decay for some time before approaching it. By increasing the distance from the radiation source. By increasing the resistance to radiation through drugs. By spending less time working with radioactive source. By absorbing the radiation in shielding material. Radiological Safety Reference Handbook (2004). 1.5 Shielding of Gamma Radiations The goal of shielding is confinement, with the eventual conversion of radioactive energy to heat which can be dissipated by cooling (air, water, etc.). The charged particle can be stop completely in the known thickness of shielding and the gamma radiation is only diminished in the intensity by any given absorber but not completely stopped. To absorb photons by the material more effectively than other any material is important so use appropriate type and thickness of shielding material to reduced the intensity of gamma radiation. The primary radioactivity particle may sometimes induce secondary energetic particles by basic interaction processes and they turn produce tertiary particles etc. It is necessary to study the dependence of 10

11 various fundamental interaction processes on particle energy and on absorber atomic number Z as well as the density. Depending on primary energy and type, various shielding methods illustrated schematically below will be appropriate. Sometimes a combination is employed, layered to deal with the successive cascade particles. Some shielding materials for various radiations depend on the energy of incident radiation is shown in figure 1.3 Fig. 1.3 Examples of some sample for various radiation shielding When alpha particle incident on the paper they cannot pass it through but the beta particles pass through it by losing some energy because of more energetic than the alpha particles but they cannot pass through the plastic. The gamma-rays easily pass through the human body or thin sheet of aluminum by losing 0the energy in the absorbed material and transmitted intensity of beam of gamma rays attenuated and is given by the fundamental law as I t I 0 e [1.4] Where, I and I o are the attenuated and unattenuated intensities of gamma beam, µ represents the linear attenuation coefficient and t is the thickness of the attenuator. This is valid only in narrow beam geometry when a collimated beam of radiation is used. In case broad beam geometry the above equation is modified as I t BI 0 e [1.5] 11

12 Where, B is the buildup factor, I and I o are the attenuated and unattenuated intensities of gamma photons, µ represents the linear attenuation coefficient and t is the thickness of the attenuator by Ervin B. P. (2010). 1.6 Literature Survey The study of linear attenuation coefficient, half value thickness, mass attenuation coefficient and total photon cross section is important in solving various problems in radiation physics and radiation dosimetry Hine G. J. (1992). These parameters are also useful in industrial, biological, agricultural and medical studies. The interaction between of incident gamma and the absorbing material depends on the incident photon energy, nature of material, atomic number or effective atomic number, the density of the material. The theoretical calculations of the mass attenuation coefficient of various elements were carried out by Hubbell J.H. and Seltzer S. M. (1995), Creagh D. C. and Hubbell J.H. (1995) and Henke et.al. (1988). The data is presented in the literature and most of the scientific communities are using the same data for comparison with the experimentally obtained mass attenuation coefficient with theoretical tabulations available in the literature, mostly with X-COM program by Berger M. J. and Hubbell J. H. (1987). There are many reports in the literature on experimental determination of mass attenuation coefficient of various elements, compound and mixtures, Basher I. I. (1997), Pawar P. P. et al (2012) and Danial S. et al (2015). It is fact that significant systematic discrepancy between theoretical and experimental values of mass attenuation coefficient even after taking all the precautions in the experiment. The factors that give rise discrepancy are the lack of proper collimation of transmuted beam, the method used for the measurements, scattering effect and calibration of instrumental. The discrepancy can be minimized by controlling the above mention parameters. The linear and mass attenuation coefficients for elements by the well-collimated beam were measured by Tupe V. A. et al (2012). The shielding properties of CaO-SrO- B 2 O 3 glasses by measuring the mass attenuation coefficient using narrow beam geometry with NaI(Tl) detector using Cs 137 Gamma source studied by Sing et al (2005). The experimental data on mass attenuation coefficient is helpful in potential applications in gamma ray shielding. The obtained experimental data compared with theoretical data. 12

13 1.7 Aim of the Present Work The knowledge of absorption of gamma rays are an important property of matter and used in many application areas of science and engineering. The study of linear attenuation coefficient, half value thickness, mass attenuation coefficient, total cross section and electronic cross section etc. of various elements by using suitable method is essential to understand their possible use in shielding purpose, industrial and medical applications. Usually, narrow beam geometry technique is helpful in obtaining accurate values of linear and mass attenuation coefficient. In the present work, measurement is done on the linear attenuation coefficient, half value thickness, mean free path, mass attenuation coefficient, total cross section and electronic cross section etc. of elements with low as well as high atomic number such as Carbon, Silicon, Sulpher, Tin, Antimony, Tungsten, Lead function of energies by using the radioisotope Ba, Na, Cs, Mn and and Bismuth as Co. The measurements of these parameters were carried out using narrow beam geometry. 13

14 References Auburn University, (2004). Radiological Safety Reference Handbook; AL, USA. Basher, I. I. (1997). Calculation of radiation attenuation coefficients for shielding concretes; Ann Nuclear energy, 24 (17), Berger, M. J. and Hubbell, J. H. (1987). Photon Cross Section on personal computer ; NBSIR, NIST, Berger, M. J., Hubbell, J. H. (1998). XCOM: Photon Cross Sections Database 8, Web Version 3.1, available at Creagh, D. C. and Hubbel, J. H. (1995). in: A. J. C. Wilsons (Ed.), International Table for X-ray Crystallography, Kluwer Academic, Dordrecht, Vol. C. Sect Danial, S., Dariush, S. and zojani, M. S. (2015). Investigating on some radiation parameter in Soft tissue; Journal of Radiational Research and Applied Science, 8, Ervin, B. P. (2010). Radiation Physics and medical Physicists, 2 nd edition; London, New York, Springer Heidelberg Dordrecht. Fig.1 and 2 are taken from the following web line on dated Feb (2016); Forshier, S. (2001). Essential and Radiation Biology and Protection; Thomson Delmar Learning, Spr. Gupta, T. K. (2013). Radiation, Ionization and Detection in Nuclear Physics, London; New York, Springer Heidelberg Dordrecht. 14

15 Henke, Davis, J. C., Gullikson, E. C. and Perera, R. C. C., (1988). LBL Report No.LBL UC-411. Hine, G. J. (1992), Phys. Revs; vol85, httpwww.franswebspace.org.ukscienceandmahsphysicsphysicsgced1-3.html Hubbell, J. H. and Seltzer, S. M. (1995). Tables of x-ray attenuation coefficients and mass energy absorption coefficients 1 kev to 20 MeV for Elements Z=1 to Z=92 and 48 additional substances of dosimetric interest, National Institute of Standards and Physics Laboratory, NISTIR5632. Khan, F. M. (2003). The Physics of Radiation Therapy, 3edition, Philadelphia: Lippincott Williams & Wilkins. Murugeshan, R. and Kiruthiga, S. (2008). Modern Physics, 14 th edition; S. Chand and Company limited. National Council on radiation protection and measurements, (1997). Deposition, Retention and Dosimetry of inhaled radioactive substances; 14. Paic, G. (1988). Ionizing Radiation Protection and Dosimetry; CRP Press. Pawar, P. P. and Bichale, G. K. (2012). Measurement of mass and linear attenuation coefficient of gamma rays of Alanine for 0.662, 1.117, 1.28 and 1.33 MeV photons, Journal of Applicable Chemistry;1 (1) Profio, A. E., (1978). Radiation Shielding and Dosimetry; John Wiley and Sons Inc. Ronald, L. K. (1995). Principles and Application of Collective dose in Radiation Protection, National Council on Radiation Protection and Measurements; NCRP report no

16 Saloman, E. B., Hubbel, J. H. and Scofield, J. H. (1988). Atomic Data Nuclear Data; Tables 38, 1. Tupe, V. A., Pawar, P. P., Shengule, D. R. and Jadhav. K. M. (2012).Total attenuation cross sections of several elements at 360 and 511 KeV, Applied Science Research; 4(6), Sing, K., Sing, H., Sharma, G., Gerward, L., Khanna A., Kumar, R., Nathuram, R. and Sahota, H.S. (2005). Gamma-ray shielding properties of CaO-SrO-B 2 O 3, Radiation Physics and Chemistry; 72 (2-3), Syed, N. A. (2007). Physic and Engineering of Radiation Detection, 1 st Academic Press Inc. Published by Elsevier. edition; Tayal, D. C. (2008). Nuclear Physics, 5 th Publishing House. edition; Mumbai, India, Himalaya Tupe, V. A., (2012). Effect of multi-energetic gamma ray irradiation on absorption coefficient of elements in the form of foils with atomic number range (4 Z 82). 16