8.1 Alpha, Beta & Gamma Radiation

Scientists working with radioactive sources need to be aware of the radiation dosage received when working with sources that are potentially harmful. 

One laboratory’s safety guidelines give indications of the minimum safe distance to work from radioactive sources. A section of these guidelines is shown in Figure 1. 

Figure 1

A freshly prepared source of Potassium-42 has an activity of 3.5 × 10⁷ Bq. Potassium-42 decays with a half-life of 12 hours, with 80% of decaying nuclei emitting β^– particles and the remaining 20% emitting gamma rays. 

To determine the dose received by a scientist working with the source the number of gamma ray photons incident on each cm² of the body must be known. 

A scientist is initially working 1.50 m from the fresh source with no shielding. 

Show that at this distance approximately 25 gamma ray photons per second are incident on each cm² of the scientist’s body.

The scientist returns 2 hours later, without shielding, and works at a distance where they receive about 21 photons per second on each cm² of their body. 

Determine whether the scientist is at a safe distance from the source according to the laboratory guidelines.

The safety guidelines in Figure 1 do not include any reference to sources of β^– emission. 

Explain why β^– emission does not need to be considered when calculating the dose of radiation the scientist receives from the source and suggest further considerations that should be included in the guidelines.

The study of beta decay provided early hints to the existence of a yet undiscovered particle. 

Scientists were interested in the energy distribution of the beta particles emitted when a radioactive source decayed. The source was known to have an activity of 120 Bq and a half-life of 10 h. 

The energy released by each decaying nucleus was labelled as X and the total energy released each second by the source was measured to be 7.5 × 10^–11 J. 

The energy distribution of the beta particles is shown in Figure 1: 

Figure 1

Explain why no beta particles had an energy equal to X. 

Determine the value of X to an appropriate degree of accuracy.

Calculate the number of atoms remaining in the source after 24 hours. 

The measured count rate of the source was 100 counts per second, which was slightly lower than the activity of the source.

Account for the difference between activity and measured count rate.

A physics undergraduate is presented with a set of data which was collected during an experiment to investigate the inverse-square law for gamma radiation. The data is shown in Table 1. 

Table 1

A note next to the data suggests the experimenter used a metre rule and a GM tube with a counter to conduct the experiment, and that background radiation was measured to be 3 counts per minute.  

The undergraduate suggests three improvements to the set of data. They propose:

• Two improvements to the presentation of experimental data in Table 1
• One improvement to Table 1 to include processed data. Two additional columns would help provide evidence to demonstrate gamma radiation follows an inverse-square law 

By considering the information provided and the data in Table 1, identify the three improvements suggested by the undergraduate.

Explain how processed data in two additional columns inTable 1 could illustrate that gamma radiation follows an inverse-square law.

Suggest an alternative approach to determining if gamma radiation follows an inverse-square law from processed data.

Figure 1 shows an arrangement used to maintain a constant thickness of a sheet of paper or steel as it is being rolled. A radioactive source and detector are used to monitor the thickness. 

Figure 1

Explain briefly how this arrangement works.

Alpha, beta or gamma sources could be selected for use in such an arrangement. 

State which source should be selected for each material, paper and steel, and explain briefly why the others would not be suitable.

State which type of radiation, , or : 

(i)         Produces the greatest number of ion pairs per mm in air. 

(ii)        Could be used to test for cracks in metal pipes.

Specific radioisotope sources are chosen for tracing the passage of particular substances through the human body. 

State and explain: 

(i)         Which type of emitting source is commonly used. 

(ii)        Why the source should not have a very short half-life. 

(iii)       Why the source should not have a very long half-life.

The exposure of the general public to background radiation has changed substantially over the past 100 years. 

State:

(i)         Two man-made sources of radiation that has contributed to this change. 

(ii)        Two sources of natural background radiation.

A Geiger counter is placed near a radioactive source and different materials are placed between the source and the Geiger counter. 

The results of the tests are shown in the table. 

 State and explain which type of radiation is emitted by the source.

A   ray detector is used to measure the count rate of a radioactive source. It has a cross-sectional area of 2.2 × 10^–3 m² when facing the source. 

Explain why the detector is not able to detect all of the radiation emitted from the source.

Two measurements of the source’s corrected count rate are recorded in Table 1. 

Table 1

Calculate the missing data from Table 1.

Alpha and beta emissions are known as ionising radiations.  

(i)         Which ionising radiation produces the greatest number of ion pairs per mm in air?Tick (✓) the correct answer in Table

                     Table 1

(ii)        State and explain why such radiations can be described as ionising.

(i)         Complete Table 2 showing the typical maximum range in air for  and particles.                

               Table 2

(ii)        State and explain which particle has a greater range in air.

 rays have a range of at least 1 km in air.

However, a   ray detector placed 0.5 m from a  ray source detects a noticeably smaller count-rate as it is moved a few centimetres further away from the source. 

Explain this observation.

Following an accident, a room is contaminated with dust containing americium which is an  − emitter. 

Explain the most hazardous aspect of the presence of this dust to an unprotected human entering the room

Radiation is a safe and cost-effective method for sterilising surgical instruments. However, some members of the public worry that irradiated surgical instruments become radioactive once sterilised.

A student detects the counts from a radioactive source using a G-M radiation detector as shown in Figure 1. 

Figure 1

The student measures the count rate for three different distances d. Table 1 shows the count rate, in counts per minute, corrected for background for each of these distances. 

Table 1

Explain, with the aid of suitable calculations, why the data in Table 1 are not consistent with an inverse-square law.

State two possible reasons why the results in Table 1 do not follow the expected inverse-square law.

Technetium (Tc-99m) is an isotope commonly used in a number of medical diagnostic imaging scans. It has a half-life of 6 hours. 

Explain why this isotope of technetium is often chosen as a suitable source of radiation for use in medical diagnosis. 

You may be awarded an additional mark for the quality of written communication in your answer.