APRIL 2001 COLLABORATION MEETING
TRINE Results JSN
Interestingly the Trine folks had problems oddly familiar to those attending the meeting. The Trine boys used 48 Canberra 10x10mm PINs (10 keV energy loss at 30 kV). They observed unexpected energy loss in PINs but didnt really study it, their solution was simply to replace the damaged PINs, They also had high voltage problems (the beam was held at 25 kV), which they attempted to solved by coating their electrodes with carbon. (It doesnt sound like this really helped, decay protons were only barely detectable above the vacuum protons). They were also forced to deal with an unexpected soft threshold in their electronics, this was solved by digitizing the wave forms to allow the use of a software threshold..
Two super mirrors gave a focused beam of superior uniformity. In all they acquired 100 days of data. Their flux at the position equivalent to our second collimator was 1´ 109 neutrons/cm2. This is about a factor of 10 higher than ours, but their overall efficiency was much lower. They recorded an average coincidence rate of 0.2 Hz per PIN as opposed to the 7 Hz we obtained with our surface barriers.
Their unpublished result is D= 6(stat.) ± 6(sys.) ± 6(stat on sys.)´ 10-4 (or about the same sensitivity as emiT) Descriptions of the apparatus and Torsten Soldners thesis can be found at:http://WWW.Physik.TU-Muenchen.DE:81/~tsoldner/trine
Improved Beam Profiles MSD
Scott outlined the new beam measuring technique that they have been using at NIST. The idea is to place Dysprosium or copper foils in the neutron beam. The resulting activity can be scanned producing intensity profiles linear over five orders of magnitude. The technology looks impressive (Figure 1 shows a beam profile taken in the lifetime trap). The only issue seems to be a smearing of the scan that the folks out there are calling blooming (Figure 2 shows the profile of an activated irregular piece of
foil) It is not expected that this will effect us much. This technique combined with a polarized 3He cell opens up the rather nice possibility of precise polarization maps. Particularly interesting is the possibility of measuring the transverse portions of spin directly.
Beam Status FEW
The cold source upgrade is scheduled to begin Aug 20th. The cold source, which will still be liquid H2 will utilize a fancy computer aided elipitical design. Additional reflectors will be placed on the exterior and the core will now be evacuated. These improvements are expected to give us a flux increase of about 1.9. The upgrade is expected to be completed (ready for us to use the beam) by 1/2002. During the shut down the NG6 folks plan to take advantage of the time to install a second monochrometer. The only impact this will have on us is that the end of the beam tube will move north by approximately two feet. Backgrounds and flux should be unaffected. We will be able to use the experimental area to set up during the shutdown, and the expected six months of running should not present a problem.
Fred proposed the idea of removing our first guide tube segment. This gives an estimated 8% flux increase and should not significantly increase backgrounds. Fred plans to provide us with an extremely detailed proposal soon.
HV tests HPM
I have made a variety of Simion simulations of the proton focusing cells. The following two plots show the potential surfaces and the distribution of incident protons that hit the proton paddle. The two smaller peaks are the protons that hit the edge of the focusing tube.
Using data based on a cylindrical beam geometry in Alejandros Monte Carlo we can model the behavior of a few thousand protons in a few seconds. I have obtained good agreement with previous results. The advantage is that we are able to quickly consider a variety of potential designs. Options that are being considered include using focusing tubes with thicker walls and the possibility eliminating the focusing tubes altogether. It seems reasonable that we can maintain the high efficiency (~90%) and at the same time reduce the field strength around the edges of the focusing tubes. We should have a decision on a test piece in a few weeks
We have also explored the effect of surface treatment of the focusing tubes. Extremely careful polishing of the tubes using a series of sequentially finer grit cloth with inspections (using a microscope) between stages led to greatly reduced field emission. Figure 3 shows the effect of polishing first the tube
from which data is being taken and then the three tubes to the sides. The remaining rate is consistent with emission from the diagonal tubes. In addition Jeff supplied us with a focusing plate constructed of thicker material and consequently having tubes with greater radii. This plate was simply electro-polished and clearly performs as well as the carefully polished plate of the original design (Figure 4).
Status of high voltage crate and cables
The new high voltage crate has been constructed and tested. It contains power supplies for SB bias, fiber optics, and preamps. A current limit on the detector bias still needs to be constructed. Commercial cables and connectors (See following photo) have arrived in Seattle and construction should be completed soon.
Si Detectors JSN
Jeff outlined the data on dead layer increases seen by the lifetime experiment. Figure 5 shows the proton energy loss after a number of trap instabilities. Each successive instability dumps a large number of charged particles onto the detector likely causing the increase in the energy loss. It should be noted that gammas show no such effect implying damage to the surface layer of the detector. Surface barrier detectors do not exhibit this trait, they either continue to operate normally or cease to function altogether.
Jeff also provided us with a reasonable explanation of why the PIPs seem to have a much higher dead layer for protons than advertised by their manufacturers. Figure 6 shows the dead layer of PIPS relative to the effective dead layer of Si (Subtracting the Au) in Surface Barriers. The PIPs are found in the upper left of the plot. (The apparent trend toward thicker Si dead layers with thicker gold layers is not understood.) The following plot shows the Boron implantation profile of a PIPS detector. After implantation about 2200 A is etched off the detector producing a widow thickness of 400 A. Clearly however the doping (and radiation damage) extends deeper. Low energy proton deposit nearly all their energy in this damaged area.
Field mapping/spin transport/MCs -KPC remotely
See Kevin's PDF report
Detector Status (proton and beta segments) HPM
We need to obtain a new power supply for beta detectors, the old one has irreplaceble failing components. The best option seems to be to adapt the SNO NCD power supplies for this use. The advantage here is that the computer interface has already been designed and code exists.
Paul Parazzolli (UW undergraduate) is examining the possibility of using a diode calibration scheme for the beta paddles. These are very stable drivers for a set of blue LEDs. We intend to steal the design (which has been tested) from LBNL. Stuart has offered his help should any obsticeles need to be removed.
Alejandro has checked all beta counters using the delta_E scintillator and a 207Bi source. The 207Bi source gives a conversion electron line that can be used for calibration. Because the 207Bi source gives many gamma rays as well, the thin delta_E scintillator is used to select betas. All PMTs were biased to approximately 2500 kV such that they would yield the same amplitude for the 207Bi source (approx 350 mV on scope). The discriminator was then set at approximately 70 keV. All rates were below100 Hz, except PMTB for Ebeta3. This PMT has been removed, and Alejandro has prepared a jig to glue a new PMT in place. Three additional phototubes have been purchased and have arrived in Seattle. One will be used to replace problem tube the others will be kept as spares. There is still some discussion as to weather or not the bases should be rebuilt. If anybody has feelings about this please let me know. Stuart has offered to have news ones constructed should it be deemed necessary.
proton segment detectors
We have set up a data base to keep track of detectors and a thickness measuring setup is in place. We have ordered SBs with dead layer of ~ 20µg/cm2 The detector ship date is 5-3-01.
UW has ordered 33 detectors, LBNL 36 detectors.
Hardware upgrades (Preamps, Shaper/ADC boards,Cooling) -HPM
Cooling (Paddle to Danger Will)
A fairly extensive redesign of the cooling system has been made. We have purchased a commercial liquid nitrogen cooling system that will allow easy change-out of the dewars and automatically gravity feeds nitrogen to the danger will heads. Gas returns through the same vacuum jacketed lines and is released. As part of the upgrade the danger will flanges are being remade with liquid nitrogen cooling in mind. See following figure. The system will easily meet the required 16 watts of cooling power. The equipment has been shipped to CENPA for testing.
We have redesigned the copper to berylia joint. The new design will use what essentially a type of solder to join the copper and berylia directly. This should avoid the berylia failures that occurred in the first run. The new berylia pieces have been made and should ship soon. We will then ship them to MRI for joining. Please see:
emiT Proton Paddle Cooling Requirements, H. P. Mumm, Oct. 2, 2000
After significant effort spent producing low power pre-amps (less than half the consumption of the previous design) we have are nearing a final design. Only high voltage tests remain, and in all other respects the current design performs extremely well. Construction will be done at CENPA and should begin as soon as the design is finalized.
Cooling of new boards
The mother board design is very similar to that used during the first run. Significant changes include contact cooling of the detectors, and cooling of the Hybrids and FETs through their leads. This cooling approach has been tested and works well enough for an experiment at a government lab. Remaining tests involve testing high voltage discharge robustness and will begin soon.
The most recent performance report is available to those who are interested:emiT NCD Shaper ADC Card Version II, Preliminary Test Results, Charles Duba, Pieter Mumm & Tim Van Wechel, July 18, 1999.
DAQ software JFW
John showed us examples of what the DAQ will look like. For those of you who have seen SNO-DAQ everything will look familiar. The idea is to present a simple graphical user interface to control the experiment. Data (Including the possibility of control) will then pass through a server to client machines anywhere on the net. Improvements include faster data rates better error handling and a more intergated approach to hardware monitoring. At the current time a great deal of progress has been made porting the SNO and existing emiT codes to Linix. Code to take singles data is available now. People interested in specifics show contact John.
DAQ hardware/integration HPM
This issue was not discussed in detail. Brian had some ideas as far as new beta electronics. I will need to contact him. John has asked people to look at various pieces of hardware, unfortunately I dont have the details at my fingertips.