168-171 - Nuclear Radiation and Gamma Absorption (CP15)

Background Counts

Put a Geiger-Muller tube on a bench, connect it to a counter, and leave the radioactive source in its store. The counter still clicks. That is not a broken detector: it is detecting background radiation.

Background radiation is ionising radiation already present in the surroundings. In a UK classroom it can come from natural sources such as radon from rocks and soil, $\gamma$ radiation from the ground and building materials, cosmic radiation from space, and small amounts of radionuclides in food and drink. Artificial sources, especially diagnostic medical exposure, also contribute to population exposure, but the key laboratory point is simpler: the detector counts background whether or not your intended source is present.

A count is the number of detections in a time interval. A count rate is:

The unit is $\mathrm{count\,s^{-1}}$ or $\mathrm{s^{-1}}$. In school radiation experiments, the useful source signal is usually the corrected count rate:

Decision point: subtract rates, not raw counts measured over different times. If the background was counted for $300\,\mathrm{s}$ and the source reading for $60\,\mathrm{s}$, the raw counts are not directly comparable until each is converted to a count rate.

Worked example:

The background count is $150$ counts in $300\,\mathrm{s}$.

With a $\gamma$ source present, the detector records $1860$ counts in $60\,\mathrm{s}$.

So:

For Edexcel, a “calculate” answer should show the conversion to rates, the subtraction and the unit. A common wrong route is to subtract $150$ from $1860$ first; that mixes a five-minute count with a one-minute count and over-corrects the source reading.

Alpha, Beta And Gamma

Alpha, $\beta$ and $\gamma$ are all ionising radiations, but they interact with matter in different ways.

The useful physical contrast is not "dangerous" against "safe". It is how often the radiation interacts while it travels. Alpha particles are massive and charged, so they interact strongly with nearby atoms. They cause many ionisations in a short distance, lose energy quickly, and therefore have low penetration. Gamma photons have no charge and no rest mass, so each photon has a lower chance of interacting over a small distance. That makes $\gamma$ radiation weakly ionising per unit path but highly penetrating.

This contrast explains why $\alpha$ radiation can be serious if an $\alpha$-emitting material is inside the body, even though $\alpha$ particles are easily stopped outside the body. It also explains why lead is used in the core practical: lead does not make $\gamma$ radiation vanish at one magic thickness, but each extra thickness gives more chance for photons to interact and be removed from the beam.

Misconception contrast: more penetrating does not mean more strongly ionising. Here, $\gamma$ is the most penetrating of the three but the least ionising per unit path; $\alpha$ is the least penetrating but the most ionising.

For an “explain” response, Edexcel credit usually depends on linked reasoning. "Gamma is penetrating" is a statement. "Gamma has no charge, so it is less likely to interact with atoms over a short distance and can pass through more material" is an explanation.

Nuclear Equations

Nuclear equations track two conserved counts:

$A$ is the nucleon number, the total number of protons and neutrons. $Z$ is the proton number, the number of protons. In a nuclear equation, the total $A$ values must balance and the total $Z$ values must balance.

Useful particle symbols are:

Worked example 1: $\alpha$ emission.

Check the nucleon numbers:

Check the proton numbers:

Worked example 2: $\beta$-minus emission.

The nucleon number stays $14$. The proton number of the nucleus increases from $6$ to $7$, because the emitted electron has charge number $-1$ and the totals must still balance:

Gamma emission is different in this bookkeeping. It carries energy away from the nucleus but has $A = 0$ and $Z = 0$, so it does not change the element:

The superscript $m$ just signals an excited nuclear state in this example; the key skill is that $\gamma$ does not change $A$ or $Z$.

Decision point: do not balance nuclear equations by chemical valency. These are not electron-shell equations. Balance the top numbers and the bottom numbers.

In beta-minus decay a neutron changes into a proton, an electron and an electron antineutrino: $n\rightarrow p+e^-+\bar\nu_e$. The antineutrino has zero nucleon and charge numbers, so including it leaves the top and bottom number sums unchanged. The bottom number means proton number for a nucleus and charge in units of $e$ for an emitted particle; an electron does not contain minus one proton. When positron emission is specified, $p\rightarrow n+e^++\nu_e$, so nuclear $A$ stays constant and $Z$ decreases by one. Use the supplied particle symbols and include the neutrino where the full decay equation is requested.

Gamma Absorption Setup

Core Practical 15 investigates how lead reduces the count rate from a $\gamma$ source. The apparatus needs a fixed geometry because count rate depends on more than the lead: it also depends on distance, alignment and detector position.

[DIAGRAM: asset_name: Gamma source, lead sheets, GM detector and counter at fixed distance with background and thickness controls; asset_slug: 049_background_radiation_gamma_absorption_core_practical__diagram_01; recommended_method: matplotlib; description: Gamma source, lead sheets, GM detector and counter at fixed distance with background and thickness controls.]