Radioactivity » Practical: Penetration of Radiation

The idea of the practical

Remember, alpha, beta and gamma are stopped by different materials. In Alpha, Beta and Gamma Radiation you learnt that fact. In this required practical you find out how to test it and how to show it with numbers.

The spec says you can do this with real radioactive sources or with simulations. Real sources are sealed, weak and handled only by a trained adult, so most students see a teacher demonstration or use a computer simulation. You can still answer every exam question by knowing the method, the results and the safety rules below.

Key terms:

• Sealed source: a radioactive source locked inside a metal case, so the radioactive material cannot escape or touch anyone.
• Absorber: any material placed between the source and the detector to stop some of the radiation.

The apparatus

Draw it as a straight line, from left to right:

• a sealed source (alpha, beta or gamma) held in a clamp or with tongs, with its opening pointing at the tube and away from people;
• a gap, where the absorbers go (paper, aluminium sheets, lead sheets);
• a Geiger-Müller tube, lined up with the source and fixed at the same distance every time;
• a counter connected to the tube, which shows the number of counts, or counts per minute (see Detecting Radiation and Background Radiation).

The method

1. With no source in the room (or the source far away in its box), switch on the counter and measure the background count for 1 minute. Repeat twice more and find the mean.
2. Set up the source, the gap and the tube in a straight line, a few centimetres apart.
3. Measure the count for 1 minute with nothing in the gap.
4. Place a sheet of paper in the gap. Measure the count for 1 minute.
5. Replace the paper with aluminium sheets, increasing the thickness (for example 1 mm, 2 mm, 3 mm and so on). Measure 1 minute each time.
6. Replace the aluminium with lead sheets, again increasing the thickness, and measure 1 minute each time.
7. Take the absorbers away. Move the tube further away in air in steps, for example 2 cm, 4 cm, 6 cm, 10 cm, 30 cm, 100 cm, and measure 1 minute at each distance.
8. For every reading, work out the corrected count rate: count rate − mean background count rate.
9. Repeat the whole test for each source.

Results: three unknown sources

A class measures a background of 24 counts per minute. They test three sources, X, Y and Z, each with the tube at the same distance. Each source has a row of measured counts and a row of corrected counts.

Source	No absorber	Paper	3 mm aluminium	Thick lead
X (measured)	504	27	25	24
X (corrected)	480	3	1	0
Y (measured)	324	318	36	26
Y (corrected)	300	294	12	2
Z (measured)	174	172	160	54
Z (corrected)	150	148	136	30

Each corrected value is the measured value minus 24. For source X with no absorber, 504 − 24 = 480 counts per minute.

Distance in air

Air is also an absorber. For a beta source the count falls over a distance of about a metre. For an alpha source it falls to background within only a few centimetres, as these example values show.

Distance from source (cm)	2	4	6	10
Corrected count rate (counts per min)	400	190	2	0

Alpha has a very short range. Gamma rays fall off only gradually with distance, and they can travel many metres in air.

Conclusions

• Alpha: stopped by paper, and by a few centimetres of air.
• Beta: passes paper, stopped by a few millimetres of aluminium, and travels about a metre in air.
• Gamma: passes paper and aluminium, and is reduced by thick lead but never completely stopped.

Safety

• Only a trained adult handles the source. Students watch a demonstration or use a simulation.
• Use tongs to pick up the source and never touch it.
• Point the source away from people, and never look at the open end.
• Keep the exposure to the shortest possible time.
• Store the source in a lead-lined box straight after use.

Exam-style question

A teacher measures the background count as 18 counts per minute. A sealed source Q is then tested with a Geiger-Müller tube. With nothing in the gap the count is 258 counts per minute. With paper it is 255. With 5 mm of aluminium it is 19.

(a) Work out the corrected count rate with paper and with the aluminium. [2 marks]

(b) State the type of radiation from Q and give a reason. [2 marks]

(c) State two safety precautions the teacher should take. [2 marks]