Med Phys 4RA3: Radiation Detectors Chapter 2

General Properties of Radiation Detectors

• Ionizing radiation detection is primarily through charge generation when radiation interacts with the detector material.
• Efficiency in measuring radiation depends on how well the radiation produces charge (electron-ion pairs) in the detector.
• Charged particles (protons, electrons) produce electron-ion pairs along their paths, while neutral radiation (gamma rays, neutrons) interacts to create secondary charged particles.

Operation Modes

• Charge Collection: A charge (Q) is generated during radiation interaction; its collection induces a current signal (i(t)) at the collection electrode.
• Detection Event Rate: Relies on the radiation field and detector efficiency; at low event rates, events are easily distinguishable.

Current Mode

• In this mode, a current meter connects to the detector output, providing a measurement of current level, typically in pA or nA.
• The response time (T) of the meter is longer than the intervals between detection events, resulting in an average current recorded at time (t).
• Used in high event rate scenarios, stabilizing the current output.

Pulse Mode

• Preserves energy and timing information of individual detection events, offering details on signal amplitude and occurrence.
• Output signal shape is influenced by the detector electronics, often modeled as an RC circuit where (R) is input resistance and (C) is summed capacitance (detector plus preamp).
• Time constant of the measuring circuit given by (\tau = RC), affecting the ability to distinguish between events.

Energy Resolution

• Critical for radiation spectroscopy, energy resolution indicates the clarity of energy measurements.
• Pulse height spectra from different detection systems highlight varying energy resolutions when measuring the same monoenergetic source.
• A peak in the spectra shows how well a system captures energy from sources, indicating the system's effectiveness in distinguishing between different energy levels.