Identify radiation patterns using absorbers

You will use a Geiger-Müller detector to count pulses from a radioactive source. Each result is the number of pulses detected during 10 seconds, not a dose measurement. You work remotely and do not handle radioactive material. The lab plays recorded experiments with real apparatus and their measured results. After a run, select or hover over each result bar to read its exact count. Record the numbered results in order, rather than estimating from bar lengths. Copy each completed result into your table before starting another run. If the lab timer runs out, return to this activity and choose Open lab again, then restore your settings and continue from your next empty row.

Alpha radiation consists of helium nuclei; beta-minus radiation consists of electrons; gamma radiation consists of photons. An alpha particle has about 7300 times the mass of an electron. Its short range is associated with dense ionisation, not simply a larger particle size. Penetration depends on radiation type and energy, as well as the material. Paper stops typical alpha particles, while beta and gamma radiation can pass through it. A few millimetres of aluminium strongly reduce many beta components. Gamma radiation is attenuated rather than stopped at one universal thickness.

A half-value thickness is the thickness that halves the transmitted intensity for a specified radiation energy and geometry. In a simplified example with a lead half-value thickness of 12 mm, a 4 mm sheet transmits more than half. This value does not apply to all gamma rays. Lower-energy gamma radiation can have a much smaller half-value thickness, so a few millimetres of lead can attenuate it far more strongly. An absorber result therefore depends on photon energy as well as radiation type.

Use your nuclear-data reference (for example Binas) or the linked Sr-90, Am-241 and Co-60 reference pages to look up the emitted radiation. Remember that a source can have more than one component. Radioactive daughter products can also contribute to a source's detected radiation.

Before measuring, predict roughly what percentage of the unshielded detector count will remain with each absorber. Treat the metal discs as approximately 4 mm thick for this prediction model. Their measured effects will be tested using the lab's named discs.

After changing a source or absorber, recheck the distance, duration and number of repeats: the lab may reset these controls.

First select basic mode, source None, absorber None, distance 15 mm, duration 10 s, and 3 repeats. Start the experiment and record the three separate results. Leave the absorber out. Calculate the mean background count: add the three counts and divide by three.

Switch to advanced mode. Set 20 mm, 10 s, and 1 repeat. For Strontium-90, measure None, Paper, Aluminium Disc and Lead Disc one at a time. Repeat those four settings for Americium-241 and Cobalt-60. Keep distance and counting time fixed.

Record one result for every combination. For each source, calculate

$$\text{percentage remaining}=100\frac{N_{\text{absorber}}}{N_{\text{no absorber}}}.$$

Use the measured counts before background subtraction for this comparison, consistently for every source and absorber. Compare small residual counts with your measured background when interpreting them.

Dose and detector counts describe different things. Dose rates may be expressed in microsieverts per hour. As a unit-conversion example, a hypothetical dose budget of 1 mSv spread over 2000 hours corresponds to 0.5 µSv/h. Converting a detector count into a dose requires calibration for the source, detector and geometry.

For this exercise, use 120 detected pulses per 10 s as a classroom comparison target. It is not a dose limit. From your own results, select the least shielding that met this count target for each source. Consider no absorber first, then paper, then the tested metal discs. If both metal discs work, aluminium is less dense than lead; thickness and total mass would still be needed for a real design.

For a simplified lead-thickness calculation, take your measured Co-60 count without an absorber as N. Assume the detected component behaves like a gamma beam with a 12 mm lead half-value thickness, and neglect background in this model. This assumption is for the calculation; it is not a dose calibration.

Optional practice: apply the same model to a gamma-only beam giving 480 counts per 10 s. How much lead would bring it to 120?

Shielded viewing windows can let staff observe a patient during medical imaging. Lead glass allows observation through a transparent radiation barrier. Its attenuation depends on glass composition, thickness and photon energy.

Use the following supplied model values for photons of energy 0.662 MeV. For glass thickness d = 1.0 cm, calculate the percentage transmitted:

$$\frac{I_d}{I_0}=\left(\frac12\right)^{d/d_{1/2}},\qquad \text{percentage}=100\left(\frac12\right)^{d/d_{1/2}}.$$

These calculations compare attenuation models. Detector counts alone cannot establish a person's dose or whether shielding is safe.