Gamma Scintillation DetectorsGamma particles are best detected with crystal scintillation detectors. The twomain type of crystals used are sodium iodide (NaI) and Germanium (Ge). A nicething about gamma detectors is that they can measure the energy of the gammaparticle. To understand how the gamma detectors work, we need to understand howthe gamma particle interacts with matter. Although the gamma particle is producedin the nucleus, when it travels through matter, it mainly interacts with electronsorbiting the nucleus. Two different types of interaction with the electrons can occur:photo-absorption and Compton scattering. We begin by discussing these two typesof gamma interactions, then we discuss the operation of the gamma detector. Photo-absorption:Photo-absorptionIn photo-absorption, the gamma is absorbed by the electron. The interaction withan electron at rest is shown graphically in the next figure.The gamma particle (photon) enters from the left with a distinct momentum andenergy, and the electron is at rest. After the interaction, and gamma particle has been”absorbed” by the electron which travels off to the right. Since energy and momentumare conserved in the interaction, the electron gains the energy and momentum of thegamma photon. Photo-absorption cannot occur with a ”free” (or unbound) electron.In order to conserve relativistic energy and momentum, there must be a third objectinvolved in the process. Thus, photo-absorption in the crystal occurs with a ”bound”electron, and the nucleus that the electron is bound to also gains a small amount ofmomentum.Compton ScatteringIn Compton scattering, the gamma scatters off the electron. The interaction withan electron at rest is shown graphically in the figure. The gamma particle (photon)enters from the left and the electron is at rest. In this case, the gamma is not absorbed,but scatters off the electron. The electron has gained some energy, and the gammaphoton has lost some. The scattered gamma photon can interact with other electronsin the material.When a photon approaches an electron, one cannot predict exactly will happen.There is a certain probability that photo-absorption will happen, a probability thatCompton scattering will occur, and a probability that no interaction will take place12at all. The angle that the photon scatters is also probabilistic. Using the principles ofquantum mechanics, one can calculate the probabilities for each of these possibilities.As with radioactive decay, probability enters in the physics of the interaction. Theprobability of each process depends on the energy of the gamma. For photo-absorptionthe probability decreases rapidly with the energy of the gamma. For higher energies,the probability for Compton scattering is much larger than for photo-absorption.The NaI Multi-Channel Analyzer (MCA)The MCA system is used to detect only gamma and X-ray radiation. However,it detects the radiation well, and the MCA can also determine the energy of gammaand X-ray particles. The MCA system consists of 3 main parts: the detector itself,the amplifier/power-supply, and a computer. The detector has two parts: a scintilla-tion crystal (sodium iodide) and a photo-multiplier tube. The computer stores anddisplays the data. For proper operation, you will need to set the high voltage of thepower supply and the amplifier gain correctly.The computer displays the data graphically. The horizontal axis is the channelnumber, and the vertical axis is the counts for the various channels. For example, inthe figure below, there are around 1200 counts in channel number 390. Channel 200has around 250 counts, and for channels greater than 420 there are very few counts.We have different types of MCA systems in our lab. Some will have a total of 1024channels, some 2048 channels, and one a total of 8096 channels. In the figure below,there are a total of 1024 channels.A nice property of the detector is that the channel number is to a very goodapproximation proportional to the energy of the gamma particle. That is, countsthat register in channel 400 have twice the energy as those that register in channel200. The scaling of the horizontal axis, i.e. the energy per channel number, dependson the amplifier gain and the voltage on the photo-multiplier tube. We will adjustthe amplifier gain to best suit the needs of our experiment. The voltage for thephoto-multiplier tube is determined by the manufacture of the detector.To interpret our data properly and identify the desired photopeaks, we need tounderstand all the features of the gamma spectrum. We will discuss these featuresvia a standard example: the spectrum of Cs137 as shown in the figure.When a gamma particle interacts with the detector a number of different outcomescan result. The gamma can be photo-absorbed by an electron in the NaI crystal, thegamma can Compton scatter off an electron in the crystal, or the gamma can scatteroff an electron outside the crystal and then enter the NaI crystal. Each of thesepossibilities gives a particular feature to the spectrum. We take each case one at atime:341.Photo-absorption in the NaI crystalThis is the ideal case, the gamma is photo-absorbed by an electron in the NaIcrystal. The electron in the crystal then acquires all the energy of the gamma particle.This energetic electron ”bounces” around in the crystal transferring its energy toother electrons. Due to the properties of the crystal, the energy goes into producingelectron-hole pairs. When the electrons fall back into the ”holes” in the crystal, a lowenergy photon (visible light) is emitted. The essential role of the crystal is to convertone high energy photon (gamma particle) into a large number of low energy photons(visible light). The visible light is then detected and measured. One can think of thecrystal as ”making change” in a bank. Suppose you had a $ 1000 bill, and wanted tobuy a candy bar. The bill is too big for the store to accept. So you go to the bankand get change: 1000 one dollar bills. Now the store can accept your money. Thecrystal changes one high energy photon into many low energy photons. We can countthe number of low energy photons with a photomultiplier tube. The nice thing aboutthe crystal is that the number of low energy photons (of visible light) is proportionalto the energy of the gamma particle.The low energy (visible) photons enter the photo-multiplier tube at one end. Thenet effect is that a current pulse is produced. The nice thing