October-November'06 Polarized Target Run with the LEGS TPC

Run Summary:

LEGS experiments in 2004 - 2006 have focused on (a) neutral-pion and (b) charged-pion photo-production experiments with polarized neutrons and polarized protons in HD. The neutral-pion measurements were completed in two runs using the LEGS calorimeter in Fall'04 and Spring'05. The final experiment, charged-pion production using tracking in a Time-Projection-Chamber (TPC) surrounded by a two tesla magnet to identify the pion charge and so separate neutron and proton reactions, was successfully completed in the Fall of 2006.

HD target #T6 was condensed and NMR calibrated on March 1st and transferred to the dilution refrigerator for polarization on March 2nd, 2006. This target was removed from the refrigerator on October 13th and transferred to the Production Dewar for polarization calibration. Transfer to the In-Beam-Cryostat for the experimental run followed on Oct. 14th. (A spare target, HD #T7, was also prepared but was not needed.)

After some initial TPC commissioning studies, data taking began on Oct. 20. A minimum of three different spin combinations are needed to sort out the contributions from polarized H, vector-polarized D and tensor-polarized D. RF transitions were used to move spin from H to D, then to flip H and finally to zero H. Data were collected with four different spin combinations:

P(H) = +57%, P(D) = +22% ;
P(H) = +26%, P(D) = +34% ;
P(H) = -10%, P(D) = +31% ;
P(H) = 0%, P(D) = +31% .

The time dependence of the target spin is shown in the accompanying plot, with blue squares for P(H) and red triangles for P(D). The horizontal axis marks time after the transfer of the target into the IBC on Oct 14th.

Data collection began at about the 140 hour mark. Negligible polarization decay occurred over the course of the experiment. The spin relaxation times were well over a year.

The polarized target was removed on Nov 11th and transferred to another dewar for further NMR measurements. Subsequent running until the Nov 19th shutdown of the NSLS was used for empty cell and calibration measurements.

Time-Projection-Chamber (TPC) for Magnetic Analysis:

A TPC was installed inside the LEGS calorimeter, SASY, to provide central tracking and separation of pions by charge. This is shown in the accompanying picture showing the upstream end of the detector.

The TPC is read out through the upstream end (green circuit boards in the picture). Surrounding the TPC are layers of trigger scintillators, the cryostat for the 2 tesla super-conducting magnet, and another layer of scintillators to veto gamma rays that shower in the magnet before entering the XTAL-BOX. The field generated by the magnet is shunted by a steel return yoke (seen in yellow in the photo).

The TPC provides snap-shots of ionizing tracks, trigged either by a surrounding annular scintillator or by a forward wall of scintillators. Ionized electrons drifted through a gradient of 12.4 KV to the back plane containing GEM foils that are read out by 7296 pads. A trigger generates the start of a time ramp, so that the time at which charge is detected at the pads is proportional to distance (z) into the TPC.

Running with each polarization combination was divided between two energy ranges, defined by laser lines and corresponding tagger settings.

• The 515 nm laser line was used to generate photons in the 190 MeV to 262 MeV range. For this, the Beth magnet was set to 1.25 tesla, which ensured that any photo-produced pion would be able to reach the TPC trigger scintillators.
• Deep-UV operation of the laser (300-333 nm) covered the higher energies ranging from 240 MeV to 422 MeV. For this mode, the Beth magnet field was set to its maximum of 1.95 tesla.

A typical (n(p) ( (-p(p) event is shown below. Colors denote charge intensity in the upper-left and time in the upper-right panels. The z-position along the TPC is plotted against x in the lower-left panel, and against radius in the lower-right panel. The target size and position are indicated by the black rectangle. (Gain variations in the pads have not yet been removed, so that tracks appear slightly jagged.) The 2 tesla magnetic field surrounding the TPC points into the screen so that positively charged particles appear to rotate counter-clockwise.

The vast majority of atomic electrons were focused forward by the Beth magnet and neither triggered the TPC nor entered its active volume. This limited the Fall'06 data set to about 0.5 tera-bites.

Summary:

The Fall'06 run has been extremely productive and will yield new data on the double-polarization observables E and G, as well as beam asymmetry ( and cross section data for the neutron and the proton. For production from the neutron, this will provide unique information needed to unravel the structure of the delta and the nucleon. The data are expected to make a significant impact and are not likely to be matched in our lifetimes.