To check whether a sample of drinking water contains a gamma emitting radioactive isotope, which instrument would be most appropriate to use?

Introduction / Context:
Environmental monitoring and nuclear safety require reliable detection of radioactive contamination in water, food and air. Gamma emitting isotopes are especially important because gamma rays are highly penetrating and can pose health risks even when the radioactive material is not ingested in large amounts. Instruments such as Geiger counters and scintillation counters are used to detect this radiation. The question asks which device would be suitable for confirming the presence of a gamma emitting isotope in drinking water.

Given Data / Assumptions:

• The suspected contaminant is a gamma emitting isotope dissolved in drinking water.
• Options include microscope, scintillation counter, lead plate, spectrophotometer and thermometer.
• We assume access to common radiation detection instruments used in nuclear science.
• The goal is to detect gamma radiation, not to examine cells or measure temperature.

Concept / Approach:
Gamma rays are high energy photons that can be detected using detectors that convert radiation energy into electrical signals or light pulses. A scintillation counter uses a scintillating material that emits small flashes of light when struck by gamma rays. These flashes are collected and converted into electrical pulses, allowing sensitive measurement of low levels of gamma radiation. A microscope is used for magnifying small objects, not for detecting radiation. A lead plate can block or shield radiation but does not measure it. A spectrophotometer measures light absorption in chemical solutions, and a thermometer records temperature. None of these substitute for a dedicated gamma radiation detector like a scintillation counter.

Step-by-Step Solution:
Step 1: Identify the type of radiation involved, which is gamma radiation from a gamma emitting isotope.Step 2: Recall that radiation detectors such as Geiger Muller counters and scintillation counters are used to detect ionising radiation.Step 3: Recognise that a scintillation counter is specifically designed to detect gamma rays by converting their energy into light and then into electrical signals.Step 4: Evaluate the other devices. A microscope provides visual magnification; it cannot detect radiation.Step 5: A lead plate can block gamma rays but cannot measure their intensity or confirm their presence in water.Step 6: A spectrophotometer measures optical absorbance and is not sensitive to gamma rays.Step 7: A thermometer records temperature and has no relation to radiation detection.Step 8: Conclude that the scintillation counter is the appropriate instrument.

Verification / Alternative check:
Practical radiation surveys of water samples often use scintillation counting, especially liquid scintillation counting when beta emitters are involved and gamma spectroscopy when gamma emitters are present. In training material for radiological protection, scintillation counters are mentioned as sensitive detectors for gamma rays and sometimes for alpha and beta particles with suitable scintillators. This strongly supports their role as the device of choice in the scenario described.

Why Other Options Are Wrong:
Option a, microscope, can reveal microscopic organisms or suspended particles but not gamma rays. Option c, lead plate, is useful as shielding, not measuring equipment. Option d, spectrophotometer, is used to measure light absorption at specific wavelengths for chemical analysis, not for ionising radiation. Option e, thermometer, measures temperature and does not provide any information about radioactivity.

Common Pitfalls:
One mistake is to think that any scientific instrument could somehow detect all phenomena. Another is to confuse optical spectroscopy with nuclear detection. Students may also focus on the word spectrum in spectrophotometer and assume it is equivalent to gamma spectroscopy, which actually uses different detectors. To avoid these errors, remember that gamma rays require specialised radiation detectors such as scintillation counters or Geiger Muller counters, not ordinary optical or mechanical instruments.

Final Answer:
To confirm whether drinking water contains a gamma emitting isotope, the most suitable instrument is a scintillation counter.