Lesson 2.1 - Scintillation Detector Basics
What is a scintillator?
A scintillator is a material that converts energy lost by ionizing radiation into pulses of light. In most scintillation counting applications, the ionizing radiation is in the form of X‑rays, γ‑rays and α‑ or β‑particles ranging in energy from a few thousand electron Volts to several million electron Volts (keVs to MeVs).
Components of a Scintillation Detector
Pulses of light emitted by the scintillating material can be detected by a sensitive light detector, often a photomultiplier tube (PMT). The photocathode of the PMT, which is situated on the backside of the entrance window (as seen in the video above), converts the light (photons) into photoelectrons. The photoelectrons are then accelerated by an electric field towards the dynodes of the PMT where the multiplication process takes place. The result is that each light pulse (scintillation) produces a charge pulse on the anode of the PMT that can subsequently be detected by other electronic equipment, analyzed or counted with a scaler or a rate meter.
Photoelctron Acceleration in a Scintillation Detector
Detection Efficiency - The Density Factor
A Well Detector
Well detectors are unique detectors that are designed to enclose the radiating source within the scintillator (except for the top). Sources may be in packaged, sealed, placed in beakers, tubes or vials, or some other form. Then the source is placed inside the cutout of the detector. Well detectors open from the top and may have a lead shield or some other mechanism for reducing background emissions.
Detection Efficiency - The Thickness Factor
When increasing the diameter of the scintillator, the solid angle under which the detector "sees" the source increases. This increases the detection efficiency. The ultimate detection efficiency is obtained with so-called "well counters" where the sample is placed inside a well in the actual scintillation crystal.
The thickness of the scintillator is the other important factor that determines the detection efficiency. For electromagnetic radiation, the thickness to stop about 90% of the incoming radiation depends on the X-ray or γ-ray energy. For electrons (e.g. β-particles) the same is true, but different dependencies apply. For higher energy particles (e.g. α-particles or heavy ions) a very thin layer of material can stop nearly 100% of the radiation.
The thickness of a scintillator can be used to create a selective sensitivity of the detector for a distinct type or energy of radiation. Thin (e.g. 1 mm thick) scintillation crystals have a good sensitivity to low energy X-rays but are almost insensitive to higher energy background radiation. Large volume scintillation crystals with relatively thick entrance windows do not detect low energy X-rays, but can efficiently measure high energy gamma rays.
Total Detection Efficiency
Silicon photodiodes (PDs) & Silicon Photomultipliers (SiPms)
Alternative ways to convert scintillation light into an electrical signal are Silicon photodiodes (PDs) or Silicon Photomultipliers (SiPms). The operation principles and different characteristics of these are discussed in a separate section. The combination of a scintillator and a light detector is called a scintillation detector.
Tech Note: Since the intensity of the light pulse emitted by a scintillator is proportional to the energy of the absorbed radiation, the latter can be determined by measuring the pulse height spectrum.
To detect nuclear radiation with a certain efficiency, the dimension of the scintillator should be chosen such that the desired fraction of the radiation is absorbed. For penetrating radiation, such as γ‑rays, a material with a high density is required. Furthermore, the light pulses produced somewhere in the scintillator must pass through the material to reach the light detector. This imposes constraints on the optical transparency of the scintillation material.