November 2nd, 2022 - How Far Away Can You Detect Radiation?
Introduction
If you hear a question like this you know something is missing. For example: What is the radioactive source and what is its activity? Also, how large is the detector and what type of detector is being used? It is obvious that sensitivity of the detector, intensity of the radioactive source and specifics about the type and energy of the radiation are of ultimate importance.
This paper will address the issues of scintillation detector efficiency versus sensitivity and some other parameters affecting detection of photon radiation (gamma-ray and X-ray spectra). The detection of Beta and Alpha radiations can be performed with attachments for the BNC hand-held instruments but this is not the focus of this paper.
Detector Efficiency
Photon radiation, such as gamma rays, are free of charge (unlike alpha and beta particles). Therefore, photon radiation must first experience a significant interaction in the detector before detection is possible. Since photon radiation may travel large distances before interaction takes place, the detector efficiency may be a small fraction of 100%. In order to describe the efficiency of a detector we need to define two classes of counting efficiency: absolute (ϵa) and Intrinsic (ϵi).
Absolute efficiency is defined as:
It should be noted that geometry and distance from the source to the detector will greatly affect the accuracy of this calculated efficiency [1]. For example, the detector must be in the form of a right circular cylinder. This is important for accurate dose rate measurements. In addition, gamma-ray energy and thickness of the detector are also factors affecting the efficiency.
The intrinsic efficiency is defined as:
The intrinsic efficiency implies that the calculation uses a solid angle subtended from the source to the detector. Therefore, the two efficiencies are related by
ϵi = ϵa(4π/Ω)
Where Ω is the solid angle between the radioactive source (S) and the detector with 4π representing the spherical radiation. The above two concepts of efficiency are important but a more practical measurement used in spectroscopy is the peak efficiency (ϵp). This measurement uses the full energy of the photopeak as shown below. The peak efficiency assumes that only the full energy photopeak events are counted. Back scattered events and Compton are not counted as seen to the left of the photopeak (1 and 2). When other extraneous counts (e.g., small scattered events) are subtracted from the total photopeak the corresponding efficiency is called the intrinsic peak efficiency (ϵip). The most common type of efficiency used for gamma-ray detectors is ϵip, also referred to as net-peak. This net-peak efficiency is important in gamma-ray spectroscopy since it can be used to calculate the absolute activity of a radioactive source.
Detector Efficiency and Sensitivity
Detector efficiency and sensitivity are certainly related when considering certain detector material properties such as atomic number (Z), light output and matching wavelength for example. Detector sensitivity on the other hand speaks more of response or degree of response to a stimulus. The stimulus in this case is the photon energy incident on the detector. A large detector that is exposed to a greater number of photons (larger solid angle of exposure) will generally exhibit a higher degree of sensitivity – assuming similar detector materials for comparison. Measurements can show that for some photon energies a smaller detector can have a higher efficiency than a larger detector when comparing different detector materials. However, the larger detector may still have a greater detection sensitivity. This can be demonstrated when comparing a smaller cerium bromide detector to a larger sodium iodide detector.
Sometimes sensitivity is the most important factor when in the surveillance mode. Obviously, you must find the radioactive source first before identification can take place when searching for the source. When in this mode one of the best choices may be a very large sodium iodide detector to maximize the sensitivity (depending upon an application like radioactive cleanup of a large area). Higher resolution detectors are also available with very efficient materials and larger volumes today. Be aware that sensitivity can be severely compromised with lanthanum materials because of intrinsic radiation from 138La and 227Ac. This limitation is represented by a high background across the spectrum [2]. This loss of sensitivity is particularly noted when sources are weak (low activity) or at a distance and therefore subject to the inverse square law [1]. In addition, shielded sources (e.g., shielded special nuclear material) may be difficult to recognize important gamma structure due to the high intrinsic background, photon absorption in the shielding and electron scatter (even though the efficiency is quite high). When the application doesn’t involve surveillance at some distance or shielding, high background and low sensitivity may not be an issue (e.g., Identifying radionuclides close to the detector).
Summary
In conclusion, it is important to use the proper term in describing a measurement or accomplishing a specific mission. For those unfamiliar with these types of measurements (e.g., geometry, eliminating measurement errors, etc.) please refer to the references given.
References
[1] Newton’s Inverse Square Law…… BerkeleyNucleonics.com/Company/Press Room
[2] J. McQuaid, The Performance of CeBr3 Detectors, BerkeleyNucleonics.com/Company/Press Room