Initial Tests of the JLab Frozen Spin Target Chris Keith Target Group Jefferson Lab September 13, 2007 Brookhaven National Lab

Physics Program with Polarized Target and Tagged Photons Approved Experiments E02-112: Missing Resonance Search in Hyperon Photoproduction E01-104: Helicity Structure of Pion Photoproduction E03-105: Pion Photoproduction from a Polarized Target Letter of Intent LOI : Photoproduction Using Polarized Beam and Target

The Conventional Hall B Polarized Target Protons (and deuterons) in 15 NH 3 ( 15 ND 3 ) are continuously polarized by 140 GHz microwaves at 5 Tesla, 1 Kelvin Used for several experiments (beam current ~ 3 nA) over a 10 month period during 1999, and Proton polarization: ~ % Deuteron polarization: ~ % 1.5 cm

The Current Hall B Polarized Target Problem: We have a “4  ” detector. We need a “4  ” target!

Frozen Spin Polarized Targets Days Polarization Two steps 1. Polarize target material (NH 3, C 4 H 9 OH, 6 LiD,...) at high field (2.5 – 5.0 T) and moderate temperature (~ 0.3 K) 2. Reduce target temperature to ~ 50 mK, and hold polarization with reduced field (0.3 – 0.5 T) The target polarization then decays exponentially during the data acquisition phase of the experiment. The target must be re-polarized (step 1) every few days.

Specifications for the Hall B Frozen Spin Target Polarizing Magnet: 5 Tesla warm bore solenoid Holding Magnet: 0.3 – 0.5 Tesla internal solenoid Target: Ø15 mm × 50 mm butanol (C 4 H 9 OH) Refrigerator: 3 He/ 4 He dilution 'fridge Q ~ K Q ~ 10  0.05 K L ~ – /s cm 2 Beam: Tagged photons Ch. Bradtke PhD Thesis, Univ. Bonn, 1999

Polarizing Magnet Max. Field: 5.1 T  B/B: < 3 × Bore: Ø127 mm Cryomagnetics, Inc. Oak Ridge, TN, USA A. Dzyubak, priv. comm..

Holding Magnet, Longitudinal Wire: Ø.1 mm multifilament NbTi, three layers Dimensions: Ø 50 × 110 Max. Field: 0.42 Tesla Homogeneity:  B/B ~

3 He/ 4 He Dilution Refrigeration XcXc XdXd - below 0.8 K, a 3 He/ 4 He mixture will separate into two phases - 3 He will absorb energy when it dissolves into the dilute phase. - if 3 He atoms are removed (distilled) from lower phase 3 He atoms from upper phase will cross the phase boundary to reestablish equilibrium - heat absorbed by n moles is: Q = n [H d (T m ) - H c (T m )] = n [94.5 T 2 – 12.5 T 2 ] = 82 n T 2 J/mol K 2 3 He Fermi LIQUID 3 He Dilute Fermi “GAS”

Continuous Dilution Refrigeration - 3 He is “distilled” from the lower, dilute phase of the mixing chamber - after distillation, the 3 He is recondensed in a LHe bath at ~1.5K and returned to mixer at elevated temperature T c - the cooling power and min. temperature depend strongly on heat exchange between the conc. (warm) and dil. (cold) fluid streams Performance of HX determines T c

Precooling unit is used to cool and condense circulating 3He

Target Cell is inserted into the mixing chamber via the load lock tube and sealed with a kapton gasket Dilution Refrigerator attaches to end of Precooler.

Refrigerator is inserted into pumping tube. 0.4T holding coil attached to 1K shield (cooled by still).

20 K heat shield is cooled by cold gas from the 4K pot

Frozen Spin Target with 5T polarizing magnet

FROST Precooler 4K Pot 1K heat exchanger 3He condenser Rear flange LHe Inlet 1K Pot

Sintered Heat Exchanger: Mixing Chamber (replaced w/ single-piece PCTFE) Still heater & level probe 5 & 20 mm copper 1.7 m long 4 m 2 surface area FROST Horizontal Dilution Refrigerator Thermometry & heater Indium seal Heat exchanger cover Still cover 3He pump tube flange and umbilical 3He fill tube

PCTFE Target Cup 15 mm x 50 mm (5 g butanol) Compression nut FROST Zero Heat Load Target Insert Aluminum beam window Kapton sealing gasket for Mixing Chamber Insert (80 K) is attached to wrench and screwed into M.C. (10 K) via load lock. Wrench removed after gasket is compressed.

Base temperature: mmols/ mmol/s mmol/s

Step 1: Polarizing Step 2: Beam On - Target is fully retracted, magnet is lifted to beam height - Target is inserted into magnet, magnet energized, microwaves on - Microwaves off, magnet off, holding coil on - Target is fully retracted, magnet is lowered -Target is fully inserted into CLAS The Frozen Spin Waltz

Summary - A frozen spin polarized target for tagged photon experiments is currently being tested at Jefferson Lab. Installation in Oct. 07! - 5 Tesla polarizing magnet is in house and field mapped Tesla superconducting holding coil (~1mm thick solenoid) has been constructed and successfully tested. Failed to reach full current during last test due to improper solder joint. Problem (hopefully) resolved. - Horizontal dilution refrigerator has been constructed and tested. Refrigerator exceeds all cooling requirements with a base temperature of 26 mK and a cooling power of 20 mW at 200 mK. - Two attempts to polarized TEMPO-doped butanol at 5T and 0.2K have been unsuccessful. Field homogeneity or low microwave power are most likely explanations. Source of field distortion (steel vacuum flange) has been replaced. Microwave power increased 20-fold. Polarization tests to continue Sept