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RADIATION SAFETY TRAINING MANUAL

CHAPTER 1
PROPERTIES OF IONIZING RADIATION

CHAPTER 1 Table Of Contents

A. STRUCTURE OF THE ATOM

Figure 1.1 Structure of the Atom
1. NUCLEUS
2. ELECTRONS

B. ATOMIC NOMENCLATURE

C. BETA PARTICLES

1. NEGATIVE BETA PARTICLES
2. POSITIVE BETA PARTICLES
Figure 1.2 Penetration Ability of Beta-Particles

D. RADIOACTIVE DECAY

FIGURE 1.3

E. GAMMA AND X-RAYS

F. OTHER MODES OF DECAY

G. BREMSSTRAHLUNG - A TYPE OF X-RAY

To better understand radiation safety procedures, a general understanding of the physical properties of ionizing radiation is useful. For a more complete description of the interaction of ionizing radiation with matter, a radiation safety textbook should be read. Consult the Radiation Safety Officer (RSO), or your Department Safety Advisor (DSA), for additional reading material.

A. STRUCTURE OF THE ATOM

The building blocks of atomic and nuclear structure are the electron, proton, and neutron. In its simplest form the atom has a dense core (nucleus) with electrons traveling in specific orbits or energy levels about the nucleus (See Figure 1.1).

Figure 1.1 Structure of the Atom

NUCLEUS

 

a. Protons - positive charge.

 

b. Neutrons - uncharged.

 

c. The mass of the neutron and proton is as follows (1 proton = 1.00727 amu; 1 neutron = 1.00866 amu). 1 amu = 1/12 of the mass of the carbon nucleus with 6 protons and 6 neutrons.

 

2. ELECTRONS

 

a. Mass equals 0.00055 amu (approximately 1/2000 of the mass of a proton).

 

b. Negatively charged.

 

c. Atom is electrically neutral if total electron charge equals total proton charge.

 

d. Electrons are bound to the positively charged nucleus by electrostatic attraction.

B. ATOMIC NOMENCLATURE

The following terms are commonly used in radiation physics:

X = Chemical symbol
A = Mass number (proton number + neutron number)
Z = Atomic number (proton number)
N = Neutron number (A minus Z)
^AX ⁶⁰Co

With the exception of ²⁰⁹Bi, all nuclei with atomic numbers greater than 82 are unstable. Many nuclei with atomic numbers less than 82 also are unstable. Unstable nuclei undergo transformations which release energy. Each transformation of a parent nucleus is called a disintegration. The disintegration rate is proportional to the number of radioactive atoms and the half-life. The half-life is the time required for half of the radioactive atoms to disintegrate. Each radionuclide is unique in terms of the type and energy of the ionizing radiation it emits, and in the duration of its half-life.

At the University of California, San Francisco (UCSF), many different radionuclides are used. Below, the classes of decay events that we most commonly encounter are identified.

C. BETA PARTICLES

1. NEGATIVE BETA PARTICLES

 

Commonly used radionuclides at UCSF (³H, ¹⁴C, ³²P, ³⁵S and ⁴⁵Ca) emit beta particles. In beta decay, a neutron is converted to a proton and an electron, and the electron is promptly ejected from the nucleus. An electron emitted from the nucleus of an atom is called a beta particle. Although the correct name for a negatively charged beta-particle is a negatron, the term is so unfamiliar that we will reserve the term "beta-particle" for a negatively charged beta-particle, and the term "positron" to denote a positively charged one.

 

Electrons emitted during beta decay have a continuous energy distribution ranging from zero to a maximum which is characteristic of a particular radioisotope. The maximum energy of a particular beta decay is defined as E_max; the mean energy (E_mean) is approximately E_max/3. The shape of the beta energy spectrum and the values for E_max are characteristic. Modern liquid scintillation counters allow the identification of radionuclides by detecting and measuring the energies of the emitted beta particles.

 

Beta-particles have a finite range in air and other materials (Figure 1.2) linearly related to the E_mean. As a rule of thumb, the range of beta particles in air is about 12 feet per MeV. For example, ³²P has an E_max of 1.7 MeV or a maximum range in air of 12 x 1.7 or approximately 20 feet. The mean range would be approximately 7 feet.

 

2. POSITIVE BETA PARTICLES

 

Some nuclei decay by beta+ (positron) emission. This decay results from a proton converting to a neutron and an electron having a positive charge (positron). The result of this type of decay is the loss of a positive charge in the nucleus of the atom. The most commonly used positron emitters at UCSF are ²²Na, ⁶⁵Zn, ⁶⁸Ga, and ¹¹⁴In.

 

A positron is an example of anti-matter. When matter and anti-matter collide, they annihilate each other, converting their mass directly into electromagnetic energy in the form of x-rays.

The positron's spectral response is similar to that of negatively charged beta-particles. When shielding positron emitters, however, they should be treated as photon-emitters, since their annihilation can result in the generation of 0.51 MeV photons.

Figure 1.2 Penetration Ability of Beta-Particles

D. RADIOACTIVE DECAY

Radioactive decay is the disintegration of the nucleus of an unstable nuclide by spontaneous emission of charged particles and/or photons. The decay rate (i.e. the number of nuclear disintegrations per second) of a radionuclide decreases as an exponential function. The activity of the sample is the curie (Ci). (See Chapter 2.)

The half-life (T_phy) is the time required for a radioactive substance to lose 50% of its activity by decay. Each radionuclide has a unique half-life (a physical property that cannot be modified). Figure 1.3 presents two graphs showing the exponential decay of radioactive gold. The half-lives of some beta-emitters and gamma-emitters are given in Chapter 5.

Clinicians and researchers must be aware of the biological half-life of a radionuclide in a biological system. The biological half-life (T_bio) is the time required for the body to eliminate one-half of an administered dosage of any substance by the regular processes of elimination. This time is approximately the same for both stable and radioactive isotopes of a particular element.

The effective half-life (T_eff) is the time required for a radioactive element in an animal body to be diminished by 50% as a result of the combined action of radioactive decay and biological elimination. T_eff is computed as follows:

T_effective = T_phy x T_bio / (T_phy + T_bio)

The biological half-life for carbon is about 10-40 days, calcium about 10⁴ days, sulfur about 100-1600 days and for phosphate about 20-1200 days.

FIGURE 1.3

Graphs showing the exponential decay of a source of 10⁸ atoms of an ¹⁹⁸Au radionuclide with a half-life of 2.70 days. The graph on the left is a linear plot while the one on the right is a semi-logarithmic one.

E. GAMMA AND X-RAYS

Gamma rays and x-rays are types of electromagnetic radiation with about the same wave length. Other types of electromagnetic radiation are radio waves, visible light, and ultraviolet irradiation. Gamma rays and x-rays are very much more energetic (have very much shorter wave lengths) than the other forms mentioned. Sometimes these rays behave like waves and have an energy proportional to their frequency. At other times they are best considered as discrete bundles of energy called photons. Gamma rays and x-rays occupy the same region within the electromagnetic spectrum. They are distinguishable by their origins - gamma-rays result from nuclear transitions (inside the nucleus) and x-rays from the interaction of electrons (outside the nucleus). Gamma rays have discrete wave lengths while x-rays cover a wide band of wave lengths.

Some of the radionuclides used at UCSF emit gamma-rays and/or x-rays. Because of their short wave length, these photons can pass through matter. As they pass through matter, they may be attenuated by their interaction with bound electrons in the matter (photoelectric effect), with "free" electrons in the matter (Compton effect), or with the transfer of their energy to the creation of a positive and negative electron pair (pair production). The importance of each of these effects depends on the atomic number (Z) of the absorbing material and the energy of the photon.

Gamma and x-ray photons with energies between 30 KeV and 30 MeV interact in soft tissue predominantly by Compton scattering. This means a partial energy transfer by the incoming photon through interaction with an orbital electron. The weakened photon continues on until it undergoes another Compton interaction. The Compton electron produces secondary ionizations by ejecting other electrons from their respective orbits. These electrons may have an energy that is higher than chemical bonds and by their interaction may alter chemical structures. The primary cause, approximately 80%, of biological damage is the result of the energetic charged particles produced secondarily by the x-ray or gamma-ray, not the original photon.

The intensity of electromagnetic radiation varies with the distance from the source according to the Inverse Square Law. This means that the intensity is inversely proportional to the square of the distance. I₁X(D₁)² =I₂X(D₂)² where: I= Intensity, D = Distance.

Example: At 1 foot a beam of gamma-rays has 1000 photons crossing a 1 cm square in one second. At 2 feet the beam will consist of 1000/2² = 250 photons/cm²/sec.

The rate at which the intensity (number of photons) decreases also depends on the density of the absorber. Lead, for example, is a more effective absorber of photons than air. If a certain thickness of an absorber reduces the intensity by 50%, twice that thickness reduces it to 25%, three times to 12.5% and so on; then the thickness of that absorber which reduces the intensity by 50% is called the half value layer (HVL). This is an exponential process; that is, as the thickness of the absorber is increased the intensity of the electromagnetic radiation decreases, but statistically never reaches zero. Note the contrast with beta particles. Beta particles can be completely shielded because of their finite path length.

F. OTHER MODES OF DECAY

There are other modes of radioactive decay besides beta, positron, and gamma decay which include alpha decay, internal conversion, electron capture, and neutron emission. Alpha decay is briefly discussed below. A textbook can be consulted to review the other modes.

An alpha particle is composed of two neutrons and two protons, and is identical to a helium nucleus. Alpha particles are emitted by many heavy radionuclides when they decay. Three familiar alpha-emitting elements are radium, uranium, and plutonium. Alpha energies range from about 4 MeV to 8 MeV. However, the range of alpha particles is very short - a 5 MeV alpha has a range of 0.034 mm in tissue and will not penetrate the skin. Although external exposure is of negligible concern, internal exposure is of very great concert. The concern is due to the very high linear energy transfer of alpha particles. Thus, extreme precautions must be taken to prevent entry inside the body by inhalation, ingestion, or skin puncture.

G. BREMSSTRAHLUNG - A TYPE OF X-RAY

Energetic beta-particles, like those emitted by ³²P, are quickly decelerated when passing through matter. The energy lost to deceleration is emitted in the form of x-rays called "Bremsstrahlung" which translates as "braking radiation". Bremsstrahlung is of concern when shielding beta emitters.

The intensity of bremsstrahlung increases with the increase in energy of the electrons or the mass of the absorbing media. Thus, it is common to use light materials, such as lucite or plastic, to shield beta particles. The shield should consist of a light material (for absorption of the beta radiation) followed by a dense material (such as lead) to absorb the bremsstrahlung.

The bremsstrahlung from a 1 curie source of ³²P solution in a glass container is approximately 10 mrad/hr at 1 foot. Approximately 1 cm of lucite is sufficient to shield ³²P.

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