Radioactivity Detection
Section: Nuclear Physics | Syllabus: Cambridge AS Level Physics 9702
Background Radiation We are constantly exposed to low levels of radiation from natural and artificial sources. Background Radiation Low-level ionising radiation that is always present in the environment from natural and artificial sources.
Sources that Make a Significant Contribution to Background Radiation You must know these four main sources: Source Description (a) Radon gas (in the air) Radioactive gas that seeps from rocks and soil; the largest single contributor to background radiation (b) Rocks and buildings Contain small amounts of radioactive elements; building materials like granite emit radiation (c) Food and drink Natural radioactive isotopes (e.g., potassium-40, carbon-14) are present in what we consume (d) Cosmic rays High-energy radiation from space; intensity increases at higher altitudes Exam Tip Learn all four sources: (a) radon gas, (b) rocks and buildings, (c) food and drink, (d) cosmic rays.
Examiners often ask you to list sources of background radiation. FIG 5.6: Sources of background radiation Pie chart showing percentage contribution: Radon gas (~50%), Rocks and buildings (~14%), Cosmic rays (~10%), Food and drink (~12%), Medical (~12%), Other (~2%).
Example: Background Radiation Variation Background radiation levels are different in different locations: Cornwall, UK: Higher than average due to granite rocks containing uranium High altitude: More cosmic radiation reaches the surface Near nuclear facilities: Slightly elevated (but still very low) Concrete buildings: Slightly higher than wooden structures Important Background radiation is always present and must be accounted for when measuring radioactive sources in experiments.
Detecting Ionising Radiation Ionising radiation cannot be detected by our senses, so we need special instruments. Key Principle Ionising nuclear radiation can be measured using a detector connected to a counter .
The detector (such as a Geiger-Müller tube) detects radiation events. Each event is registered by the counter, which displays the count. Radiation Detection Instruments Geiger-Müller (GM) Tube and Counter The most common instrument for detecting and measuring ionising radiation.
FIG 5.7: Geiger-Müller tube cross-section Thin mica window for detecting alpha particles, Metal tube (cathode, negative), Central wire (anode, positive), Argon gas at low pressure, High voltage supply, Counter display, Arrows showing radiation entering through window.
How a GM Tube Works Radiation enters the GM tube through a thin window Radiation ionises the argon gas atoms inside the tube Electrons are attracted to the positive central wire (anode) Positive ions move toward the negative outer cylinder (cathode) This creates a pulse of electric current Each pulse is counted and displayed on the counter A click sound is produced for each count Remember The GM tube detects individual radiation events.
More clicks per second means higher radiation intensity. Features of a GM Tube Thin mica window: Allows alpha particles to enter (thick windows would block them) Dead time: Brief period after each detection when the tube cannot detect another event (~100 microseconds) Cannot distinguish: Between alpha, beta, and gamma radiation Detects presence: But doesn't measure energy of radiation Count Rate The activity of a radioactive source is measured using count rate.
Count Rate The number of radiation counts detected per unit time. Units of Count Rate Count rate is measured in: counts/s (counts per second) counts/min (counts per minute) Exam Tip The syllabus specifies these units: counts/s or counts/minute .
Always include units in your answers. Example: Basic Count Rate Example 1: Basic Count Rate A detector records 600 counts in 1 minute. Count rate = 600 counts/min Or: 600 ÷ 60 = 10 counts/s Example: Converting Count Rate Units Example 2: Converting Units A source has a count rate of 15 counts/s.
What is this in counts/min? Count rate = 15 × 60 = 900 counts/min Corrected Count Rate Because background radiation is always present, we must subtract it to find the true count rate from our source. Corrected Count Rate The count rate from the radioactive source alone, with background radiation subtracted.
Formula Corrected count rate = Measured count rate − Background count rate Experimental Procedure Measure background radiation: Take readings with no source present for several minutes Calculate average background: Find the mean background count rate Measure with source: Place radioactive source near detector and record total count rate Subtract background: Calculate corrected count rate using the formula above Example: Calculating Corrected Count Rate Example 3: Calculating Corrected Count Rate Step 1: Background radiation measured over 5 minutes = 150 counts Background count rate = 150 ÷ 5 = 30 counts/min Step 2: With radioactive source present = 450 counts in 5 minutes Total count rate = 450 ÷ 5 = 90 counts/min Step 3: Corrected count rate = 90 − 30 = 60 counts/min This means the sou…
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