Building a Radiation Detector
Project Description
As a final project for my Principles of Radiation Detection course, each student was tasked with the assignment of creating a radiation detection device with the capabilities of detecting either a certain type of radiation or multiple types of radiation. At the end of the assignment, each person was to present his or her methods of detection, as well as evidence of the device’s success at detecting radiation. This project was unique because we were able to decide not only what type of detector we would create, but also how we would deliver the evidence that the detector functioned properly. Drawing on what I had learned in class, I decided to design a liquid scintillation detector that would allow me to detect thermal neutrons. Using vials of liquid scintillation fluid doped with increasing amounts of LiF, my goal was to find a peak concentration of LiF in which the reaction between thermal neutrons and Li-6 was neither limited by the concentration of LiF, nor inhibited by the cloudiness of the solution.
Background Information
Scintillation detectors detect radiation using a photo-multiplier tube (PMT) that detects scintillating photons emitted by the medium, in this case an organic scintillation fluid composed of hydrocarbons. The scintillating photons are produced when the medium is excited by ionizing radiation and the energy absorbed by the medium is released as light. To detect thermal
Fig. 1. Fission reaction between Li-6 and thermal neutron [1].
Detector Design
For my project to be successful, it was imperative I devised the proper environment for my detector to properly function. PMTs are incredibly sensitive, and any photons released by a source other than the scintillating fluid can skew the results. With the help of a GA, I found an appropriate PMT (Figure 2) and fit a cardboard tube snugly over the PMT, capping the other end and completely blocking out the light from the environment. To ensure photons of a higher energy would not broach the enclosure, I wrapped the cardboard tube in aluminum foil, to reflect environmental photons (Figure 2), and draped a thick, black blanket over the tube in order to absorb environmental photons before reaching the tube (Figure 2). Using a moderator and Cf-252, which naturally produces fission products, including fast and thermal neutrons. I induced the fission reaction
neutron activity, the thermal neutrons would interact with an isotope with a high absorption cross section that would trigger a fission reaction, releasing ionized particles. One such isotope is Li-6, which releases an alpha particle and a tritium particle, both of which will excite the scintillation fluid. By mixing LiF into the scintillation fluid, I allowed thermal neutrons to interact with natural Li-6, thereby releasing ionizing particles and exciting the scintillation fluid.
Fig. 2. Photo-multiplier tube, aluminum wrapped cardboard tube, and experimental setup.
with thermal neutrons and Li-6 in the scintillation fluid and recorded the measurements. Due to the many variables associated with the detection, there was no way to determine an accurate energy scale for the spectrum, however the results showed the reaction as a small mound at higher energy that grew with increased LiF concentration in the fluid (Figure 3).
Fig. 3. Results of experiment as concentration of LiF increases.
Challenges towards Design
Throughout the course of designing the detector I was faced with unexpected barriers that caused me to make tough decisions about the design of my detector. For example, I soon discovered that LiF was not highly soluble in my scintillation fluid. To combat the issue, I added water to a few vials of the scintillation fluid with the intention of increasing the amount of LiF dissolved into the solution. The idea was successful to an extent; however, water is a thermal neutron moderator, indicating that increasing the water content could inhibit the thermal neutrons from reacting with the Li-6. After doing some trials with water, I determined it was more effective to not add water to the vials.
Project Conclusion
To complete the project, I presented my findings to my peers in a 15-20-minute presentation, where I described the detector, the obstacles I encountered throughout the project, my results, and the improvements I could make on the detector. Though challenging, tackling this project was exciting. I was able to utilize my engineering design process, especially during the research and testing phases, which was particularly satisfying.
[1] "Nuclear Equations," [Online]. Available: http://genderi.org/nuclear-equations.html. [Accessed 18 04 2019].