Gamma-Gamma Correlations in Na^22 and Co^60

Textbook reading

  • Read section 8.4: The Scintillation Counter.
  • Read p.409-421 (section 9.5), the description of 22Na experiments.
  • Also examine the diagram on p.341.

I. Introduction

This experiment is concerned with the operation of high speed pulse circuitry, the coincidence technique, the angular correlation of gamma rays in 22Na and 60Co [and in a later lab the annihilation lifetime of the 22Na positron in a solid material].

II. Scintillation Counters

1. With the plastic scintillator and NaI(Tl), check the difference between the pulses with and without the source.

2. Plastic Scintillator: Observe pulses with the organic scintillator (mounted on a different PMT). Start at -1000 V and increase the HV until pulses are visible on the scope.

DO NOT EXCEED -2000 VOLTS ON THE PMT!

3. NaI(Tl) Scintillator: Observe the pulses from the photomultiplier with the inorganic scintillator. Using the 22Na source begin at a tube voltage of -1000 V and increase until the pulses are visible on a scope.

DO NOT EXCEED -2000 VOLTS ON THE PMT!

Estimate the length of the pulses for both types of scintillator.

4. Plateau curve: Send the pulse from one of the tubes with plastic scintillator through the discriminator and into the scaler. Measure the counting rate as a function of HV both with and without the 22Na source. Subtract the dark count from the signal and plot. Determine the correct operating voltage for the tube and discriminator setting. Repeat for the other tube.

5. Delay curve: With the two counters viewing the 22Na source at 180 degrees, send the signals through the delay box. Measure the counting rate as a function of the delay between them for both signs of the delay. Compare the width of this curve with the pulse widths from the discriminators. Set the delay at the optimum value.

III. Gamma Angular Correlation

1. Measure the singles rates in the two PMT’s. Using the width of the delay curve measured above as the resolving time estimate the accidental counting rate from Racc = R1*R2*DT. Measure the accidental rate by placing the two tubes at 90 degrees and putting a long delay in one channel (why?).

2. Determine the true coincidence rate as a function of angle. Calculate the expected curve from the geometry of overlapping circles and compare with your results. Why is there a flat background uncorrelated in angle in the data?

3. Make a series of measurements at three different distances from the source. In the coincidence experiment, what are the effects of moving the two plastic scintillators closer to the 22Na source? How does the width of the coincidence peak change ? How close can they be placed if the scintillators are at 90 degrees? What causes this limitation?

IV. Angular Correlation for 60Co

Now make a series of measurements for the two photons from the 60Co source. Compare the angular correlation from 60Co and 22Na by evaluating the parameter α = [C(180)-C(90)]/C(90), where C(theta) is the number of coincidences at angle theta. Evaluate the error and central value for the parameter alpha for both 22Na and 60Co.

[Note: You may need to take data for longer periods at each angle for the 60Co source.]

V. Self Test

  1. Briefly describe the functions of a discriminator, a delay box, a scaler and a coincidence unit.
  2. How does a scintillator material work? How does a photomultiplier work? (See Appendix E.2 of the text)
  3. How does one plateau a photomultiplier? Sketch the expected shape of the curve.
  4. How does one measure a delay curve? What does it tell you? Sketch a typical delay curve.
  5. Where do the three gamma rays in this experiment come from? Why are two of them correlated in angle? (See p.341 of the text)
  6. What should the angular correlation curve for 22Na look like? Sketch it. (Remember there is a flat background)
  7. Explain the formula for the accidental coincidence rate.
  8. In the coincidence experiment, what are the effects of moving the two plastic scintillators closer to the 22Na source? How close can they be placed if the scintillators are at 90 degrees? What causes this limitation?
  9. Qualitatively compare the angular correlations from the 22Na source and from 60Co.

VI. Technical Notes:

  1. For the inorganic crystal, use negative HV from the FRONT(!) MHV connectors on the HV supply. For other phototubes, use negative SHV (red cables). Be careful when using the FLUKE supplies, there is HV on the front panel at the MHV connector.
  2. The set values on the HV panel for the upper supply do not correspond to the actual values. These can be determined correctly from the meter on the front.
  3. Note for photons we do not expect a flat plateau curve. (here the photoelectric effect and Compton scattering determine the shape). This is in contrast to charged particles.
  4. Scalar channel A is set up for negative pulses while channel B works only for positive pulses (used in another experiment).
  5. Don’t set the discriminator pulse width to be too short i.e. less than 10 ns.

August 17, 2018

Tom Browder