Wednesday November 6, 2013

We will meet in TED 2561B on Wednesday November 6 at 3:00 pm EST.

For those calling in we'll use the ReadyTalk audio conference system.

1. Dial Toll-Free Number: 866-740-1260 (U.S. & Canada)
2. Enter 7-digit access code 4402297, followed by “#”

Agenda:

1- New "5 MeV" Dipole update media:new_5Mev_dipole_Oct25.pdf media:new_5Mev_dipole_Oct25.doc

2- Bubble Chamber progress at Argonne

3- Discuss the Bubble Chamber dead time: rates and systematic error. How long does it take to acquire one event?

4- Now we have an Accelerator Operations Logbook, BUBBLELOG: [1]

5- Under Accelerator Task List, ATLis, there is now Project: Bubble Chamber to assign tasks to.

6- There will be an Accelerator Seminar on Thursday January 23, 2014:

 - Title: Measurement of 16O(γ,α)12C with a bubble chamber and a bremsstrahlung beam at Jefferson Lab Injector
 - Abstract: media:Abstract_Acc_Seminar.pdf media:Abstract_Acc_Seminar.docx

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Notes from this meeting:

• New Dipole Magnet:

 - We are going with Model 3 in Jay's design: with 14.6 cm wide, 10 cm long poles
 - For about $10K one can buy a Hall probe system with claimed accuracy 0.01% and temperature coefficient 10 ppm/C
 - For a 12.5 cm long magnet, the beam scrapes the beamline
 - Do we have the steel on-site?
 - Working with Shaun to complete magnet design and finalize dimensions
 - This new magnet will work fine up to 16 MeV/c (the maximum energy of the new upgraded Injector)

• Bubble Chamber progress:

 - JLab Facility rejected our request for wall power needed for the new Chiller (Two pole, three wire 208V, NEMA 6-15 plug, 8A).
They do not provide this configuration. Their normal procedures are to provide 120 V, 20 A, NEMA 5-20 receptacles or 208/120 V,
30 A, NEMA L21-30 receptacles.

• Bubble Chamber dead time:

 - In the proposal, at each beam energy, we need to collect about 600 events to achieve a statistical error of about 4%.
   We assigned 10 hours for each energy, i.e., 1 event/min.
 - After the bubble formation, and being registered by the camera, the pressure is increased very fast (40 ms) to its high value
   to quench the bubble. The pressure stays at the high value for few seconds to completely quench the bubble. Then the
   pressure is reduced to superheat the liquid again and this takes about 5 seconds. Thus, in all, it takes about 10 seconds to
   acquire 1 event. The time it takes the pressure to decline is the main source of uncertainty in the dead time. At HIGS, this
   uncertainty was found to be 0.9 seconds.
 - There is minimum and maximum rates where within this range the bubble chamber will be able to measure the cross section with
   good accuracy.
 - The minimum rate:
        - Is determined once the background rate is known. The real event rate must be higher than the background rate. For example
          if the real events are equal to the background events, i.e., we acquire 1200 events (half of them are background),
          then the background subtraction will increase the statistical error by √3 to 7%.
        - At HIGS, the background rate was caused by cosmic rays and was about 1 event/min. This cosmic-ray background is expected
          to be different at JLab tunnel due to earth shielding (soil: 13 ft, concrete: 2 ft).
        - Cosmic rays trigger the chamber in two ways: the muons hit a nuclide in the superheated liquid and the nuclear fragments
          trigger the chamber, or secondary neutrons (produced by cosmic rays) elastically scatter inside the superheated liquid.
          We hope to reject the neutron events with the acoustic signal but they still add to the overall rate.
 - The maximum rate:
        - Is determined from the dead time and its uncertainty: τ ± dτ. At HIGS, this was: 10 ± 0.9 seconds.
          And from the goal of keeping the dead time systematic error contribution to the yield less than 2%.
        - In this case, the maximum rate is: Rmax = 0.02 / dτ = 0.02/0.9 = 1.3 events/min. This gives a live time (lt) of:
          lt = 1 - Rmaxτ = 78%. Thus, the dead time correction is 1/lt=1.28 with a corresponding systematic error of 2%.
 - What to do? 
        - Reduce the uncertainty in the dead time (dτ).
        - Reduce the background rate and have the means to identify and reject the background events.