Nuclear Physics Laboratory Gamma spectroscopy with scintillation detectors M. Makek Faculty of Science Department of Physics Zagreb, 2015 1
1 Introduction The goal of this excercise is to familiarize with one of the most common detectors in high-energy and nuclear physiscs experiments - the scintillation detector. In addition students will learn basics of gamma radiation spectrosopy using this detector. The scintillation detector consists of a scintillation material and a photomultiplier. The scintillation material has a specific property of producing tiny light flashes called scintillations, when being traversed by a high energy ionizing particle, such as alpha, beta or gamma particles. In order to efficiently detect the light flashes the scintillation material is typically coupled to a photomutliplier tube (PMT). The role of the PMT is to convert the scintillation photons into electric signal and to amplify it so that it can be effectively measured. The scintillation detector can measure the energy deposited by the particle in the scintillation material, as well as the time of impact and the goal is to measure these two parameters with the highest possible precision. The following sections describe the principles of operation and types of scintillation detectors, following with introduction to gamma spectroscopy techniques, description of the particular experimental setup and tasks to be performed by the student. 2 Scintillation detectors 2.1 Scintillators The scintillation materials are materials that produce light scintillations where being traversed by a high-energy ionizing particle. When passing through the material, an ionizing particle interacts via Coulomb interaction with the surrounding atoms. These interactions are dominantly with electrons, since interaction with nuclei have a much smaller probability due to the small nuclear cross section. The incoming particles originating from radioactive sources that are typically detected in a nuclear laboratory have energies of the order of 1 MeV. A particle with such energy can easily excite atomic electrons with typical energy gaps between the states of 1-10 ev. The excited electrons will decay after a short time to the ground state emmitting photons typically in the visible or ultra-violet part of the spectrum. There are two general categories of scintillator materials: inorganic scintillators and organic scintillators. In this laboratory excersise we will use the former type and thus explain more details regarding its physical priciples. 2
2.1.1 Inorganic scintillators The luminiscence properties of inorganic scintillators, i.e. the deexcitation by emission of visible or UV photons, are direct consequence of their crystal structure and not the property of single atoms. Hence these materials will only have scintillating properites in the solid crystal state. In general in inorganic crystals, the atomic electrons interact with neighboring atoms and the electons of the neighboring atoms interact with the second neighbors etc., hence the whole crystal becomes a single system. This causes atomic levels to split by very small amounts in order to obey the Pauli principle. Hence instead of having well defined energies the outer electrons form energy bands called valence band and conducting band, as depicted in Figure 1. When an ionizing particle passes through scintillator material it interacts with atimic electrons. As the electrons from the valence band receive energy through this interaction they are excited to the conduction band. After a certain relaxation time characteristic of each material, the electron decays back to the valence band emmitting a photon. In ideal crystals the energy of this photon is equal to the energy of the gap between the valence and the conduction band, hence the photons have a high probability to be reabsorbed in the crystal. Such crystals would not be suitable for particle detection, since the created photons are practically trapped. A characteristic of the inorganic scintillators that distingushes them from other inorganic crystals is the presence of luminiscence centres in their crystal lattice. These centres are ions of an impurity that either substite one original ion (substitutional impurities) or they are placed between the original ions in the crystal (interstitial impurities). The impurities introduce energy levels inside the forbidden gap between the valence and the conduction band and as such play crucial role in detection properties. If an excided electron from the conduction band is caught by such a level in the gap and decays raditively afterwards, the energy of the emmitted photon will be smaller than the gap energy, and hence it will not be resonantly absorbed, but will traverse the crystal. 2.2 Photomultipliers A photo-muliplier tube (PMT), depicted Figure 2 is a device which used to convert photons (e.g. originating from luminescence) to electric current. It is basically a vacuum tube, enclosed by a glass window on one side to allow the scintillation photons to enter. The photons hit a photocathode where they produce electrons via photoelectric effect. The electrons are then accelerated in the electric field, first towards focusing electrode and then towards the 3
Figure 1: A schematic of the band structure of an inorganic scintillator. first dynode. A dynode is an electrode which acts as anode and cathode at the same time. When hit by an accelerated electron it emmits secondary electrons which are accelerated towards the next dynode and finally collected at the anode. In this way a single photon can produce a measurable electric current. Figure 2: A drawing of the photo-multiplier tube connected to a scintillator. 3 Experimental setup The basic components of the setup are depicted in Figure 3. The experimental setup for this excercise consists of a NaI(Tl) scintillation detector coupled to a photomuliplier-tube. The detector is enclosed in lead shielding to reduce the influence of the environmental radioactivity. The high-voltage 4
to the PMT voltage divider is supplied from a HV unit placed in the NIM crate. The PMT anode signal is lead to the preamplifier, which provides signal amplification as well as the proper impedance matching between the detector and the 50 Ohm NIM electronics. The preamplifier output signal is further amplified and shaped in a Gaussian shaping amplifier in the NIM crate. The fully amplified and shaped signal is digitized by the Canberra Multiport ADC. The Mutliport is connected to a personal computer via USB and specific software. Figure 3: Schematic drawing of the experimental set-up. 4 Assignments 1. Set up the experiment (to be performed only under supervision of the teacher): (a) Assemble the experimental set-up according to the given scheme. (b) Turn the NIM crate power switch ON. (c) Set the photomultiplier high voltage to 1000 V. Take care not to exceed this limit. (d) Examine the signals from the PMT anode, preamplifier and amplifier using digital oscilloscope. (e) Turn on the computer and start the Genie2000 program. 2. System calibration (a) Place the 60 Co source on top of the scintillation detector and close the Pb shield (performed by the teacher). 5
(b) Set the signal amplitude using coarse and fine gain controls on the amplifier, such that the higher photo-peak is centered around 80% of the total scale. (c) Set the software discriminator threshold using Genie2000 functions to reject only the noise. (d) Measure the spectra of known sources 241 Am, 60 Co for 10 minutes each. (e) Measure the background spectrum for 30 minutes. (f) Determine the calibration curve energy vs. channel using photopeaks (background subtracted) from the known sources. 3. Measurement and analysis (a) Measure the spectrum of the unknown source for 10 minutes. (b) Describe all the observed shapes in the spectrum and determine characteristic energies and underlying processes. (c) Determine the full-width-at-half-maximum ( E) of all the measured photo-peaks. Determine the dependence of the relative energy resolution E/E vs. E and explain the obtained result. 4. Written report - provide a written report according to the given instructions. 6