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Jack Wang
1) What is the photon counter?
2)
Why only single photons? If intensity of bunch is proportional to signal, why single photons?
3) What are you measuring: “location” through time? Z position of bunch?
4) How good is the timing?
5) What is the “cold finger”
6) How good does the optics have to be
This summer I will be working with Jeff Corbett at the Light Shack for the SPEAR3 Ring. What we
have in mind is building a single photon counting setup to map the longitudinal distribution as well as
energy of all electron bunches inside the SPEAR ring.
The Light Shack takes the synchrotron radiation from the SPEAR ring and filters out the harmful
wavelength radiation with a cold finger, which is a piece of metal that blocks out x-ray radiations. As a
result from the cold finger, only radiation in the visible light wavelength (with a minimal amount of
inferred) enters the light shack through a series of guide mirrors. Taking this visible light beam, using a
series of optical devices, I will focus the beam and direct it into a light-tight box where the photo
detector sits. Using a special computer to gather data from the photon counter, we will be able to map
out the electron bunch distribution based on the amount of photons emitted.
This setup sounds easy to do, but there are many specific difficulties that must be addressed
during the experiment. Many difficulties root from the fact that we have to use a single photon counter
for our purposes, which means any light beam of more than a few photons will overload our counter.
We have planned 2 ways to solve this problem. One is building a light tight box with a small hole to allow
the synchrotron radiation beam to hit the counter. This would block out all the ambient light in the lab
room. Another aspect of addressing the problem is filtering down the incoming light beam. The light
beam we have coming from SPEAR contains billions of photons. This beam, if unfiltered, will destroy out
detector. Therefore we will be using both uniform density filters and band-pass filters to filter down the
beam into single photon range.
After all the setup is ready, a chain of calibration of the detector is needed to produce good
results. This calibration includes synchronizing the detector time reference frame with the orbital period
of the electron bunches. Many other similiar calibrations are needed in order to produce meaningful
results. From the results, we will be able to tell the location of each electron bunch from when they
produce photons that hit the counter, and we will be able to measure the energy carried by each
electron bunch by how many photons from this bunch hit the detector.