Lab B4: The Creation and Annihilation of Antimatter

1. Lab B4: The Creation and Annihilation of Antimatter SFSU Physics 490 Spring 2004 Professor Roger Bland

2. Lab Partners • Yvette Martinez • Michael Hoffman • Elizabeth Manrao

3. Experiment Description • In this experiment you can observe evidence for production and absorption of gamma and beta rays, and for the creation and annihilation of antimatter! Processes involving gamma rays will be interpreted using Feynman diagrams.

4. Learning Goals • Understand the sodium-iodide scintillation detector. • Learn how a pulse-height analyzer works. • Learn the different ways in which gamma rays interact with matter. • Understand the energy-level and decay schemes for cobalt-60.

5. 3 Ways Gamma Rays Interact • The Photo-Electric Effect • Compton Scattering • Pair Production Experimental Physics, Dunlap, Oxford 1988 pg. 282.

6. The Photo-Electric Effect Definition: The emission of an electron from a surface as the surface absorbs a photon of electromagnetic radiation. Electrons so emitted are termed photoelectrons. Source: http://ctd.grc.nasa.gov/dictionary/p.html

7. Compton Scattering Definition: The scattering of photons from charged particles is called Compton scattering after Arthur Compton who was the first to measure photon-electron scattering in 1922. Source: http://www2.slac.stanford.edu/vvc/glossary.html

8. Pair Production Definition: An absorption process for X-ray and gamma ray radiation in which the incident photon is annihilated in the vicinity of the nucleus of the absorbing atom, with subsequent production of an electron and positron pair. Source: http://roland.lerc.nasa.gov/~dglover/dictionary/p.html 

9. Experimental Procedure Equipment List • Tracerlab Detector #45292 • Computer: Halley 2 (IP 130.212.16.76) • Oscilloscope: SN# 23042030, dual trace • Canberra 816 Amplifier & High Voltage Power Supply #70501 • Radioactive sources: Co-60, Cs-137, Na-22

10. Experimental Procedure Diagram of Apparatus

11. NaI Scintillation Detector • Scintillation detectors detect light emitted by electrons when they change energy levels. • We use ionizing radiation from radioactive sources to provide the electrons sufficient energy to move into a higher energy shell. • The electrons do not remain in the higher energy for long. Right away they fall back to their original level and, as they do so, they emit photons of visible light. • The number of photons of light emitted, and the intensity of the light, is proportional to the energy of the incoming radiation. • Scintillation detectors are used to detect radiation and to separate out the energies.

12. Scintillation Detectors • Photomultiplier tubes are necessary in scintillation circuits to convert photons of light from the scintillator into electrical pulses. • They are also used to amplify the size of the original signal. • The incident radiation interacts with the crystal to produce a light photon. This light photon then hits a photocathode. • The energy from this light photon is absorbed by an electron in the light sensitive material and this electron gains enough energy to leave the photocathode. • The ejected electron forms the basis of the electrical signal and is amplified at approximately four electrons for every electron. • The high voltage power supply provides the stability the circuit needs in order to operate consistently.

13. NaI Scintillation Detector Source: http://www.physics.isu.edu/radinf/naidetector.htm

14. Multi-Channel Analyzer • The pulses leaving the scintillation detector have amplitudes proportional to the energy which the particles or photons deposit in the detectors. • These pulses are sorted according to their height. This is equivalent to sorting the particles or photons according to their energy. • Electronic systems which do this are called pulse height analyzers (PHA). Single channel PHA's only count pulses of a given amplitude. Multichannel analyzers (MCA's) can scan a whole energy range and record the number of pulses they count in each of the channels.

15. A Look at Cobalt 60 • Cobalt (Co) is a metal that may be stable or unstable (radioactive, man-made). The most common radioactive isotope of cobalt is cobalt-60. • Cobalt-60 is used in many common industrial applications, such as in leveling devices and thickness gauges, and in radiotherapy in hospitals. Large sources of cobalt-60 are increasingly used for sterilization of spices and certain foods. The powerful gamma rays kill bacteria and other pathogens, without damaging the product (cold pasteurization).

16. Cobalt 60 Isotope diagram

17. Spectrum of Cobalt 60

18. Summary & Conclusion • Using the Isotope diagrams we expected to find 1.17 MeV, 1.33 MeV and 2.50 MeV peaks on the MCA. After calibrating the MCA we indeed found evidence supporting this. • We also found evidence for Compton scattering.