Chapter 2. Radiation

Chapter 2. Radiation • Radioactivity 2.Radiation interaction with Matter 3.Radiation Doses and hazard Assessment

2.1 Radioactivity • Overview • Types of Radioactive Decay • Energetics of Radioactive Decay • Characteristics of Radioactive Decay • Decay Dynamics • Naturally Occurring Radionuclides

c) Beta Decay Spectra and Neutrino ? Pauli: Neutrino with spin 1/2 is emitted simultaneously with beta, carrying the missing energy.

c) The mass of the neutrino is negligibly small.

d) Positron Decay Energy

3）36CI decays into 36S (35.967081 u) and 36Ar. If the energy release is 1.142 MeV to 36S and 0.709 MeV to 36Ar, calculate the masses of 36CI and 36Ar. Describe the modes of decay. • 5) The radionuclide 41Ar decays by β-emission to an excited level of 41K that is 1.293 MeV above the ground state. What is the maximum kinetic energy of the emitted β-particle?

Radioactive Decay Kinetics -exponential Number of radioactive nuclei decrease exponentially with time as indicated by the graph here. As a result, the radioactivity vary in the same manner. Note lN = A lNo = Ao

6) The activity of a radioisotope is found to decrease by 30% in one week. What are the values of its (a) decay constant, (b) half-life, and (c) mean life?

b) Three Component Decay Chains

Daughter Decays Faster than the Parent λI < λ2, transient equilibrium: daughter's decay rate is limited by the decay rate of theparent. • λI << λ2, The activity of the daughter approaches that of the parent. This extreme case is known as secular equilibrium(久期平衡).

4）An initial number NA(0) of nuclei A decay into daughter nuclei B, which are also radioactive. The respective decay probabilities areλAand λB. IfλB = 2λA , calculate the time (in terms of λA)when NB is at its maximum. Calculate NB (max) in terms of NA(0)

2.2Radiation interaction with Matter overview Photon Interactions Neutron Interactions Interaction of Heavy Charged Particles with Matter Scattering of Electrons in a Medium

1) overview

I = Io e–μx • mean-free-path length • Half-Thickness

4) Interaction of Heavy Charged Particles with Matter Fast moving protons, 4He, and other nuclei are heavy charged particles. Coulomb force dominates charge interaction. They ionize and excite(give energy to) molecules on their path. The Born-Bethe Formula for Energy Loss of Charged Particles.

能量损失

Range of Heavy Charged Particles in a Medium  source Shield Particles lose all their energy at a distance called range.

A material is found to have a tenth-thickness of 2.3 cm for 1.25 MeV gamma rays, (a) What is the linear attenuation coefficient for this material? (b) What is the half-thickness? (c) What is the mean-free-path length for 1.25-MeV photons in this material? • The specific rate of energy loss (-dE/ρdx) of a 5 MeV proton in silicon is 59 keVmg-1cm2 and its range R' is 50 mg cm-2 . Calculate values of (-dE/ρdx) and range R' for deuterons, tritons, 3He and a particles, all of which have the same speed as the proton.

2.3 Radiation Doses and hazard Assessment • Historical Roots • Dosimetric Quantities • Natural Exposures for Humans • Radiation Effects

Historical Roots Early workers exposed to X-rays developed dermatitis（皮炎）. Uranium miners developed skin lesions. People working with radioactivity experienced illness. Researchers exposed to radioactivity suffered radiation sickness at advanced age. Manhattan project workers in Los Alamos, Oak Ridge, Hanford, and atomic worker in the former USSR suffered anorexia（厌食）, fatigue, headache, nausea（反胃）, vomiting, and diarrhea.

Collective Response to Radiation Risk In 1928, the International Committee on X-ray and Radium Protection was formed to look into the risk of radiation. It is now called International Commission on Radiological Protection, ICRP. In 1942, a group of health physicists had the responsibility to assess problems and implement safe operation procedures regarding radioactivity. After WW2, the (American) National Council of Radiation Protection (NCRP) was formed in 1946. Guidelines are given for radioactive material handling and applications. Today, safety committee is set up to deal with radiation risks.

Mission Statement of the ICRP The International Commission on Radiological Protection, ICRP, is an independent Registered Charity, established to advance for the public benefit the science of radiological protection, in particular by providing recommendations and guidance on all aspects of protection against ionising radiation. From www.icrp.com check with ICRP for up-to-date guidance regarding radiation

Protection standards GB4792-84 放射卫生防护基本标准卫生部发布 GB8703-88 辐射防护规定环保局发布 GB 18871-2002 电离辐射防护与辐射源安全基本标准 2002-10-08 发布 2003-04-01 实施中华人民共和国国家质量监督检验检疫总局发布

Lord Kelvin 2) Dosimetric quantities a physical measure correlated with a radiation effect.

Radiation Absorption and Dosage typeunits RadioactivityBq, Ci    Exposure dose Gy, rad (R) Quality factorQ Biological doseSv, rem The amount of energy absorbed from exposure to radiation is called a dose. The radiation effect measured by a dosimeter reflects an equivalence of certain dosage of X-rays. The amounts are defined in certain units as shown here.

Units for Radiation Source (review) The SI unit for radioactivity is Bq (1 becquerel = 1 dps). The decayis not necessary all absorbed unless it’s internal. 1 curie = 3.7e10 Bq. These units have nothing to do with energy, type (a, b, g, X-rays, neutrons, protons or particles), and effect of radiation. Commonly used units megacurie kilocurie millicurie microcurie nonocurie picocurie these modifiers are also used for other units. disintegrations per second the fluence is not closely enough related to most radiation effects to be a useful determinant.

Dose Units - roentgen, rad, and gray Amounts of absorbed energy are not the same as exposed.The amount of radiation energy absorbed is called a dose. A roentgen ( R) is a dose of X- or -rays that produce 1 esucharge at STP (negative and positive each or 2.1e9 ion pairs) in 1.0 L. 1 R = 352.1e9 = 7.35e10 eV (*1.6x10-12 erg/eV) = 0.12 erg (per 0.00123 g air) = 1 rad(100 erg per g of any substance) 1 Gy = 1 J / kg (1 J per kg of any substance is a gray, Gy) = 1e7 erg / kg = 100(100 erg/g)~100 rad average energy In air, the average energy required to produce an ion pair is 35 eV photons corpuscular radiation 1 Gy being equal to an imparted energy of 1 joule per kilogram.

A Dosage Evaluation Example A 5-MeV  particle is absorbed by 1 gram of water, estimate the dosage in rad and rem. The Q factor is 10 for  particle, and thus the dose is 8e-7 rem or 8e-9 Sv. If the a particle is absorbed by a of 10-9 g cell, then the dose is 109 times higher (0.8 Gy, 8 Sv), exceeded lethal (致命) dose for most living beings.

Integral Dose Used in Radiation Therapy • Total energy absorbed by an organ called integral dose is gram-rad or g-rad or g-Gy total dosage received by an organ. • g-Gy = dose * mass of the organ • Accumulated dose is the dose received over a period, but g-Gy is the total dose received in a single time.

The Quality Factor QF and Dosage Units The factor reflecting the relative harmfulness of various types of radiation is called the quality factor (QF) or relative biological effectiveness (rbe) Biological dose = QF * exposure dose

Exposure and Biological Dosage SI unit cgs unit Exposure unit 1 Gy = 100 rad (=100 R) Biological dose 1 Sv = 100 rem (= Qrad) Gy: gray, Sv: sievert, R: roentgen, rem: roentgen equivalent man

Summary of Units for Radioactive Dosage QuantitySymbolSI unitcgs unitConversion factor radioactivity ABq Ci 1 Ci = 3.7e10 Bq exposure dose XC/kg R 1 C/kg = 3876 R absorbed dose DGy (J/kg) rad 1 Gy = 100 rad =6.24 eV/g biological dose HSv (QF*Gy) rem 1 Sv = 100 rem C/kgcharge produced by exposure to radiation

Effective Dose Equivalent In a human, different organs have different radiological sensitivities, To account for different organ sensitivities and the different doses received by the various organs a special dose unit, the effective dose equivalent HE, is used to describe better the hazard a human body experiences when placed in a radiation field.

Tissue weighting factors adopted by the ICRP [1977] for use in determining the effective dose equivalent.

Naturally occurring radionuclides in the human body deliver an annual dose to the various tissues and organs of the body as follows: lung 36 mrem, bone surfaces 110 mrem, red marrow 50 mrem, and all other soft tissues 36 mrem. What is the annual effective dose equivalent that a human receives?

Kermakinetic energy of radiation absorbed per unit mass 比释动能 indirectly ionizing (uncharged) radiation If Etr is the sum of the initial kinetic energies of all the charged ionizing particles released by interaction of indirectly ionizing particles in matter of mass m, then

total moss interaction coefficient the linear energy absorption coefficient μtri which account for fewer secondary photons escaping from the interaction site, are sometimes encountered. (a) Energy deposition for photon energy involved in the interactions in an incremental volume of material, (b) Formulas for the energy per unit mass of the material in the incremental volume, corresponding to the various energy increments in (a), (c) Linear coefficients defined by their proportionality to the mass energy relationships in diagrams (a) and (b).

Photon Kerma and Absorbed Dose If, at some point of interest in a medium, the fluence of radiation with energy E is Ф, the kerma at that point is f(E) is the fraction of the fraction of the incident radiation article's energy E that is transferred to secondary charged particles μ(E)/ρis the mass interaction coefficient for the detector material. μtr(E)/ρ for charged secondary particles and excludes the energy carried away from the interaction site by secondary photons一定物质对特定能量的间接致电离粒子的质量能量转移系数。

Example What are the iron kerma and absorbed dose rates from uncollided photons 1 meter from a point isotropic source emitting 1014 5 MeV gamma rays per second into an water medium? the total mass interaction coefficient for 5-MeV photons is found to . The uncollided flux density 1 meter from the source is,

Example : What is the dose equivalent 15 meters from a point source that emitted 1 MeV photons isotropically into an infinite air medium for 10 minutes at a rate of 109 photons per second? neglect air attenuation over a distance of 15 m QF = I 0.15 μSv

Dosimeters for Dosage Monitoring Dosimeters are devices to measure exposed doses. Film-badges, electroscopes, ionization chambers, biological and chemical dosimeters have been used for radiation monitors. Plants, cells, bacteria, and viruses reacting to radiation are biological dosimeter candidates. Ferrous sulfate, FeSO4, solution is a chemical dosimeter due to the reaction: 4 Fe2+ + energy + O2 4 Fe3+(brown) + 2 O2- Some glasses and crystals serve as solid state dosimeters. Shelf life, linearity, stability, usage simplicity, easy-to-read, dose-rate and equal responses to various radiation are some considerations.

Chemical 3-dimensional Dosimeter Ferrous ions, Fe2+, are oxidized by ionizing radiation, and convert to ferric ions, Fe3+, which complexes with xylenol（二甲苯酚） orange dye to give an orange compound. When the sample is prepared in a gel form, it serves as a 3-dimensional dosimeter, because the complexes are localized in the gel. These dosimeters are useful for planning radiation medical treatments such as radiation cancer treatment.

3) Natural Exposures for Humans

Radioactivity in Nature 222Rn is responsible for higher levels of background radiation in many parts of the world. because it is a gas and can easily seep out of the earth into unfinished basements and then into the house Radon The uranium decay series.

Summary of the annual effective dose equivalents from various sources of natural background radiation in the United States. Source: NCRP [1987].

Chapter 2. Radiation