Which scientific instrument is commonly used to measure and detect levels of radioactivity?

Introduction / Context:
This question belongs to nuclear physics and applied instrumentation. Radioactivity refers to the spontaneous emission of particles or electromagnetic radiation from unstable nuclei. Measuring and detecting the intensity of this radiation is important in medical applications, radiation protection, nuclear industry and scientific research. The question asks you to identify which device among the options is specifically designed to measure radioactivity.

Given Data / Assumptions:

• The task is to find an instrument that detects and measures radioactive emissions.
• The options include mass spectrometer, cyclotron, nuclear reactor and Geiger Muller counter.
• Each of these devices has a specific function in nuclear or atomic science.
• We assume standard textbook definitions of these instruments.

Concept / Approach:
A Geiger Muller counter, often simply called a Geiger counter, is a radiation detection instrument that uses a Geiger Muller tube filled with gas. When ionising radiation passes through the tube, it ionises the gas, causing a brief conductive discharge that is detected electronically. The device counts these discharge events to give a measure of radiation intensity. In contrast, a mass spectrometer measures mass to charge ratios of ions, a cyclotron is a particle accelerator used to accelerate charged particles to high energies, and a nuclear reactor is a device where controlled nuclear fission reactions occur. Only the Geiger Muller counter is specifically intended for detection and measurement of radioactivity in the field or laboratory.

Step-by-Step Solution:
Step 1: Recall the purpose of a Geiger Muller counter. It is designed to detect ionising radiation such as alpha, beta and gamma rays. Step 2: The device works by counting pulses created when radiation ionises gas within the Geiger tube. Step 3: These counts can be converted into a reading of radiation intensity in counts per second or similar units. Step 4: A mass spectrometer, by contrast, is used for analysing the composition of substances and determining the masses of ions, not for routine radiation monitoring. Step 5: A cyclotron is an accelerator that produces high energy charged particles, which is very different from a detection instrument. Step 6: A nuclear reactor is a power generating or research device where nuclear fission reactions are maintained, not a detector. Step 7: Therefore, the correct instrument for measuring radioactivity is the Geiger Muller counter.

Verification / Alternative check:
Think of images or descriptions from textbooks and media. When people refer to a handheld device that clicks faster near a radioactive source, they are describing a Geiger counter. Radiation safety officers often carry these devices to measure exposure levels. In laboratory exercises on radioactivity, students frequently use Geiger counters to record counts from a radioactive sample. None of the other devices listed is portable or used in this manner for direct radiation measurement. This confirms that the Geiger Muller counter is the correct answer.

Why Other Options Are Wrong:
A mass spectrometer requires careful vacuum and electromagnetic systems and is used for chemical analysis, not general radiation monitoring. A cyclotron is a large and complex machine used to accelerate particles and is not used as a detector. A nuclear reactor is a site where radioactivity is produced but is not itself a measuring instrument. While all of these devices are connected to nuclear or atomic physics, they do not serve the specific role of measuring ambient radioactivity in the way a Geiger Muller counter does.

Common Pitfalls:
Students sometimes choose nuclear reactor because they associate it strongly with radiation, forgetting that the question asks for a measuring device, not a source. Others may confuse mass spectrometry with radiation detection because both involve charged particles and detectors. To avoid such confusion, always focus on the primary function of each device. Remember that the Geiger Muller counter is the classic instrument used to detect and measure radioactivity, often represented with characteristic clicking sounds in demonstrations.

Final Answer:
The instrument commonly used to detect and measure radioactivity is the Geiger–Muller counter.