Electron-positron Annihilation

1. Electron-positron Annihilation Experimentation and theoretical application in Positron Physics Veronica Anderson, David Tsao, Mark A. Rodgers COSMOS, UC Santa Cruz

2. Overview • The purpose of our project and experiment is to measure (accurately) the mass of a positron through the gamma-ray detection of the annihilation between the positron and an electron

3. Antimatter • In 1932 British physicist Paul Dirac derived the equation E2=m2c4+p2c2 • This equation tells us that for every type of particle of ordinary matter (e.g. protons and electrons) a particle of opposite charge and certain quantum properties exists, called antimatter • Antimatter particles are rarely found in nature, and are therefore difficult to detect • Antimatter is usually created in the modern universe during nuclear decays

4. Positrons • Positrons are the antimatter equivalents of electrons • Positrons have a mass equal to that of an electron, but have an opposite charge and certain quantum properties different from those of the electron Artist’s conception of the Lawrence Livermore National Laboratories’ electron linac1, used for positron detection

5. Electron-positron Annihilation • When matter and anti-matter collide, they destroy each other in a flash of high-energy radiation • The gamma rays emitted in the annihilation are easily detectable • In an electron-positron annihilation the g-rays are emitted linearly • Sometimes the electron and positron form a brief bond through the electromagnetic force, creating Positronium • When electron-positron pairs first form Positronium, the direction of the energy emitted from their annihilation will not be linear

6. Positron detection • The most practical method for detecting positrons is measuring the gamma emission of their annihilation with electrons • Many larger experiments utilize particle accelerators, but we used simpler apparatus Photograph of the first detection of the anti-electron (positron), in 1932 at CERN http://athena-positrons.web.cern.ch/ATHENApositrons/wwwathena/anderson.html A large facility used for positron detection (http://wwwpat.llnl.gov/H_Div/Positrons/PositronFacility.html)

7. Objective • Our primary objective was to detect the gamma-ray (g-ray) emission of the annihilation during the collision of the positron and electron • From the g-ray emission we hoped to determine the mass (m) of the positron itself • Since Einstein’s equation E=mc2g tells us that mass and energy are interchangeable, the energy of the emission should directly correlate with the mass of the positron • We also hoped to apply what we have learned about particle physics and anti-matter to early universe astrophysics, particularly the baryon asymmetry problem

8. Apparatus • We used a 22Na positron source for our experiment. 22Na emits most positrons (b-rays), in addition to a small number of gammas. • We used a stationary aluminum plate of about .5 cm thickness to prevent positrons from directly striking our detector • Our detector was a Sodium Iodide Scintillator • The entire experiment was encased in lead bricks with dimensions of about 5x10.5x20 cm Sodium Iodide Detector2(http://detectors.saint-gobain.com/Media/Documents/S0000000000000000003/oper 20manl 200304.pdf) We fed our data into a Canberra Multi-channel Analyzer, and then transferred the data onto a computer for analysis

9. Our Apparatus Scintillator Digital Oscilloscope Lead Shielding Canberra Below Screen -24 V Power Source High Voltage

10. Canberra 35-Series Multi-channel Analyzer Our Positron Data on the Analyzer

11. Calibration • To calibrate our Sodium Iodide Scintillator, we used two g sources of known energy levels: 137Cs and 133Ba • The Cs isotope emitted a smooth radiation curve with a peak intensity of 662 KeV • We also took a measurement with no source to determine what our background distribution would be, and we found that our Scintillator was picking up very little noise Our apparatus in calibration

12. Noise Reduction

13. Data Collection • We collected our data using a Canberra 35-Series Multi-channel Analyzer • We compared the data we received with the noise and with a calibration sources--the Barium isotope did not emit the spectrum we had expected to see • The image at right is displaying the data that we received for the gamma emission of the cesium source, with which we compared our eventual positron (22Na) annihilation energy Our Canberra Multi-channel Analyzer displaying Cesium data

14. Positron Data • This is a graph of the number of counts at each energy level of the photons emitted from the Sodium-22 source • The intensity peaks at about 537 KeV

15. Analysis • We used several techniques for analysis of our data • Using a computer program we wrote, we attempted to analyze the energy of the detected gamma-rays • Although the program failed to produce results, we were able to make the calculations by hand • We found that the intensity of the energy of detected gamma-rays peaked at 537 KeV • Theoretical values have the positron with a mass-energy of about 511 KeV • Several discrepancies may account for the difference in energy detected and the mass of the positron, especially the KE of the positron (and possibly the electron as well)

16. Calculations E=mc2 E=537.0 keV (8.603x10-14J), c2=8.988x1016 m=(E/c2) Mass (m) in kg=(8.603x10-14)/(8.988x10 -16) m=9.561x10-31kg Theoretical mass= 9.1095x10-31 kg

17. Conclusions • Using the value we measured for the energy of the gamma-ray emitted from the annihilation, we were able to calculate the mass of the positron in kg • We calculated that the mass of the positron is 9.5499x10-31 kg, which is spectacularly close to the accepted mass of the electron: 9.1095x10-31 kg • Although we were slightly off from the mass we had hoped to find, our error was very slight--we didn’t take KE or thermal energy into account in our calculations

18. Acknowledgments We have not, fortunately, conducted this project alone. Among those to whom we owe the greatest thanks are Paul Graham and Dave Dorfan, our spectacular physics professors. Other thanks: Department of Physics, University of California, Santa Cruz Department of Physics, University of California, Berkeley Santa Cruz Institute for Particle Physics Fred Kuttner John and Jason Steve Kliewer Stuart Briber Nuri and Jessica (and the rest of the COSMOS staff)