Objectives

1. Distinguish between the terms isotope and radionuclide.

2. Name and identify the possible products of radioactive decay.

3. Briefly outline the procedures for measuring radioactivity. Include the principles and procedures for using the Geiger Muller, Gas Flow, Liquid Scintillation and gamma Scintillation counters.

4. Define the following terms:

• Counts Per Minute (cpm),disintegrations per minute (dpm), efficiency, background.
• Curie (Ci), millicurie(mCi), microcurie (µCi)
• Specific radioactivity.
• Radioactive decay, decay constant (sigma), half life (T 1/2).

5. Given the values of appropriate constants, perform calculations to interconvert curies, decay constants, half lives, cpm, dpm, and efficiency, or explain why such calculations are not possible.

6. Describe the radioisotopes approved for use in the laboratory in which you will work with respect to the following properties:

• decay scheme
• type and energy of emission
• half-life

Resources (Keyed to Objectives):

1. Isotope, radionuclide

The atomic nucleus contains protons and neutrons. A nuclide is a nucleus which contains specified numbers of neutrons and protons; over 1500 nuclides have been recorded. Nuclides that have the same number of protons, but different numbers of neutrons are known as isotopes. Certain combinations of neutrons and protons are unstable and break up spontaneously, a process referred to as radioactive decay. Such nuclides are called radionuclides or radioisotopes.

For example, three types of Hydrogen exist:
 

2. Products of radioactive decay

Beta Particle (e-)

A e- particle is an accelerated electron ejected from the nucleus by the following process: Neutron; Proton + Electron (e-) + antineutrino. The antineutrino is a particle having no charge and negligible mass.

Alpha particle

An alpha particle is a helium nucleus, i.e., 2 protons + 2 neutrons. It has a positive charge and is only found as a decay product of elements of high atomic weight.

Gamma rays

Gamma rays are not particles; they are a form of electromagnetic radiation(i.e., photons)having wave lengths shorter than those of ultraviolet light. Gamma rays are identical to high energy x-rays, but from a different source.
 

Characteristic Properties of Alpha, Beta, and Gamma Radiations

Emission	Character	Chargea	Massb	Velocityc	Penetration
Alpha	He2+	2+	4.0026	about 0.1 speed of light	stopped by paper or human skin
Beta	electron (e -)	1 -	0.000548	up to 0.9 speed of light	several mm in human tissue
Gamma	short x-rays	0	0	speed of light

a The unit charge employed here is the amount of the electron itself.

b The mass is expressed in atomic mass units.

c The velocity of light in a vacuum is 3.00 X 10¹⁰ cm/sec.

Certain radionuclides produce more than one type of emission, e.g., both beta and gamma rays.

3. Procedures and principles for measuring radioactivity

Geiger-Muller counter

A G-M counter is based on the principle that the radiations produced by radioactive decay will ionize gases, i.e., they are ionizing radiations. The gas used is usually an inert gas such as argon, helium, or neon containing about 0.1% of a halogen (bromine or chlorine) which acts as a quenching agent to prevent the self perpetuating ionization that would otherwise occur. The gas is held in a chamber which contains electrodes maintained at a potential difference to collect the ion pairs which result from the interaction of the ionizing radiation with the gas. As the ion pairs are collected, pulses are produced which, under suitable conditions, give a measure of the radiation. G-M counters are commonly used to monitor radiation.

The G-M tubes used for monitoring must be fairly rugged for use around the laboratory, and the window required prevents the passage of alpha particles and beta particles with energies less than 0.25 Mev electron volts such as those from 3^H. They are, therefore, useful only for monitoring gamma radiation and beta particles with energies greater than0.25 Mev such as those from ³²P. The G-M tubes of some G-M counters are covered with a thin mica window which allows less energetic beta emitters such as ¹⁴C to be detected.

Gas Flow counter

The principle of the gas flow counter is similar to that of the G-M counter. The gas flow counter has a constant supply of gas and can have a thin window or even no window at all. Thin window or windowless gas flow counters will detect Alpha particles and even the weakest beta emitters.

Liquid Scintillation counters

Liquid Scintillation counting is the method of choice for measuring beta emitters, particularly weak beta emitters. The sample is dissolved or suspended in a solution which contains an organic chemical which fluoresces when acted upon by ionizing radiation. The radiation is thus converted into pulses of light which are detected by a photomultiplier. The amount of light produced is related to the energy of the beta emitter and, by employing suitable gain and window width settings on the amplifier, emitters having different energies can be distinguished. It is, therefore, possible to use two or more beta emitters in a single experiment if their energies of emission are sufficiently different.

Gamma Scintillation Spectrometry

X-rays pass freely through the solutions used for liquid scintillation counting. They are therefore detected by using solid fluors containing atoms of high atomic numbers. Perhaps the most commonly used solid scintillator is a sodium iodide crystal containing thallium ions as an intentionally-added impurity. Impinging rays excite electrons which produce light when they return to their ground state. The light is detected by a photomultiplier.

4. Definitions of Terms:

Dpm, cpm, efficiency, background

The term disintegration refers to an ejection of a single alpha particle or beta particle from a nucleus. The nuclear event resulting in the production of an alpha or beta particle may also result in the production of a gray from an excited nucleus which results.Cobalt-60 decays according to the following equation:

60	Co g Ni* + beta
27

Where the asterisk indicates that the resulting nickel atom is in an excited state.

The excited nickel-60 atom then emits a high energy gamma ray as it goes to the ground state according to the equation:

Ni* g Ni + gamma

overall the decay of Co is as follows:

Co g Ni + beta + gamma

The rate at which these events occur is expressed as disintegrations per minute (dpm).

These disintegrations can be recorded by a suitable counting device with various degrees of efficiency. If you knew by some independent means that your sample had 1000 disintegrations per minute but your counter only recorded 500 counts per minute, you would conclude that you were counting with 50% efficiency.

Therefore, cpm/dpm X 100%= Efficiency

The efficiency with which sources can be counted is very variable and depends on the nature of the radionuclide, its chemical and physical state, and the type of counter you are using. Some counters count some sources with efficiencies close to 100%.

Every counter will register some counts even when the counted sample contains no added radioactivity. This is called background and is caused by cosmic rays, natural radioactivity, radioactive fallout, and electronic noise in the circuitry of the machine. The true cpm of a sample is therefore (sample cpm - background cpm) and the efficiency is more correctly:

sample cpm - background cpm X 100% = efficiency dpm

The Curie

This is the basic unit in which radioactivity is measured and expressed. It originally was defined as the radiation produced by 1g of pure radium. It is now arbitrarily defined as 2.22 X 10¹² dpm. A curie is inconveniently large and you will deal in millicuries and microcuries which have the following values:

Unit	Curies	Disintegrations per second (dps)	Disintegrations per minute 9dpm)
Curie (Ci)	1	3.7X1010	2.22X1012
Millicurie (mCi)	1X10-3	3.7X107	2.22X109
Microcurie (uCi)	1X10-6	3.7X104	2.22X106

Millicuries and microcuries will be the quantities used by the Radiological Laboratory Supervisor in ordering radiochemicals and are the quantities you will have to deal with when maintaining inventory records.

Specific radioactivity

Specific radioactivity is the number of curies or dpm per unit weight of a radiochemical.

For example, the units may be dpm/mg or dpm/micromole. This value is used when radiotracer experiments are done, and it is necessary to calculate how much of a substance is represented by a given number of cpm.

Example:

In an experiment to study the rate of protein synthesis by E. coli (¹⁴C) leucine having a specific radioactivity of 4 X 10⁴ dpm/µmole was included in the nutrient medium. After a 1 h incubation, protein isolated from the E. coli contained 5000 cpm/g protein. Calculate the rate of incorporation of leucine into the protein of E. coli.

Calculation:

Incorporation of 4 X 10⁴dpm = Incorporation of 1 µmole.

Incorporation of 5000 dpm=	1 X 5000 µmoles
	4 X 104

Rate of leucine incorporation=	1 X 5000 µmoles/g protein/h	=0.125µmoles/g protein/h
	4 X 104

d. Radioactive decay

Radioactive decay is a first order reaction. This means that the rate of decay is independent of the number of atoms present if large populations of atoms are considered, which is the case even with the low levels of activities used in tracer experiments. It also means that a certain fraction of nuclei will decay in any given time. This fraction is called the decay constant and its units are time, -1.

Rate Equations:

Equation: N=N_oe^{(-lambda) (t)}
Where N=activity at time t
N_o = original activity (at time zero)
 

Lambda=	Decay Constant=	.693
		T1/2

Example:

A sample of ³²P had 1000 dpm at 0 time. What will its activity be after 5 days. T1/2 = 14.28 days

Substituting N=	1000e- X	.693 (5)
		14.28

N=1000e X -0.242647

N=1000(0.784548)

N=784.5dpm

Half-life:

A very useful piece of information is to know how long it will take a source to decay to half its original activity.

Equation:

N=N_oe ^{(-lambda) (t)}

N	= e (-lambda) (t)
No

but	N	is 1/2 because activity is half its original value.
	No

Therefore, 1/2 = e ^{(-lambda) (t)} where t = T 1/2

taking logs, 1n 1/2 = (-lambda) (T 1/2)

- 0.693 = (-lambda) (T 1/2)

0.693 = (lambda) (T1/2)

and lambda=	0.693
	T1/2

Exercises.

To check your ability to manipulate equations try the following exercises.
 

1. The half-life of ⁵¹Cr is 27.8 days. What will the activity of a ⁵¹Cr sample of 5000 dpm be 7 days later?

Answer: 4199 dpm

2. A sample of ⁶⁰Co having an original activity of 20,000 dpm was found to have an activity of 18, 725 dpm after 6 months. What will its activity be after 2 years?

Answer: 15,369dpm

You also understand the relationship between curies, dpm, cpm, and efficiency. Combining your talents will enable you to perform the following types of calculations.

1. A 0.01 µci source of ³⁵S (T1/2 87.9 days ) was counted in a liquid scintillation counter after 200 days and was found to contain 2600 cpm. Calculate the efficiency of the counting system

(1 µci = 2.22 X 10dpm)

Answer: 57%