Laser Driven H/D Target at MIT-BatesChris Crawford, Ben Clasie, Dipangkar Dutta, Haiyan Gao, Jason Seely

http://ldt.mit.edu

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RpEX Experiment

The H/D nucleus also gets polarized through the hyperne interaction. In a strong eld, the nuclear and atomic spin decouple, weakening the hyperne interaction; however, the high H/D density in the spincell increases the number of collisions to compensate, and the system is designed to be in spin temperature equilibrium. The graph to the left shows the electron and nuclear polarization of D as a function of the spin temperature in equilibrium. For H, the electron and proton have the same polarization.

LNSLABORATORY FORNUCLEAR SCIENCEMIT

Polarized Source PolarimeterIntroduction

Laser Driven Target Results

Spin Exchange Optical Pumping

Studies of atomic fraction vs dissociator aperture diameter

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Atomic fraction at the target cell using the new dissociator.

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RF dissociator

spincell where K is optically pumped and transfers polarization to the H/D atoms and nuclei through spin-exchange collisions

potassium ampoule

support for spincell oven

transported tube and hot air jacket rest on top of target cell

The H/D gas is sampled through a 3 mm hole in the side of the storage cell close to the transport tube. A second hole for sam-pling is located 15 mm off center to ensure that the gas is not being directly sampled from the transport tube. The entire polar-

imeter is mounted to the target chamber through a bellows and gimble mount for alignment. Blank copper gaskets with 3 mm holes in the center are placed between each stage for beam collimation and dif-ferential pumping.

The rst stage houses a 1 Tesla permanent sextupole magnet for spin-state separation. The magnet is 15 cm long with a 5 mm inside diameter. This chamber is pumped with a Varian Starcell 300 l/s ion pump to 10-7 Torr.

The second stage is pumped to 5×10-9 Torr with a 300 l/s ion pump and NEG sorption pump. The beam is detected with a Balz-ers Prizma QMA with an electron multi-plier (SEM), modied to output an analog signal. To enhance the signal, the beam is chopped at 20 Hz, and the signal passed

through a lockin amplier. The resultant signal to noise ratio is about 50:1. This setup is used to measure both the atomic fraction and the polarization. The atomic fraction is obtained by measuring the percentage drop in the mass two signal as the RF dissociator is turned on, after subtracting the background. The polariza-tion is measured by the drop in the mass one signal when the laser shutter is opened to allow optical pumping.

In the last decade, several groups have demon-strated the feasibility of polarized H/D targets by spin-exchange optical pumping as opposed to the more traditional atomic beam sources. The atomic fraction and polarization are signicantly lower than an atomic beam source; however, because of active pumping the ow rate can be 25 times greater allowing for a potentially higher overall gure of merit. A great candidate for such a target is the new BLAST detector package currently being installed in the South Hall Ring at the MIT-Bates Linear Accelerator Center. It is designed to study the spin-dependent electromagnetic

response of few-body nuclei using a longitudi-nally polarized electron beam at energies up to 1 GeV. We have been working to develop a stable laser driven target ready for installation in BLAST in the near future. Our goal is to achieve 60% atomic fraction and 50% polariza-tion at a ow rate of 2×1018 atoms/s.

BLAST

The spin-exchange optical pumping technique involves transferring angular momentum from a circular polarized laser beam to potassium and subsequently to H/D via spin-exchange colli-sions. The potassium 4 2S1/2, me= -1/2 ground state is excited by a σ+ photon to the excited 4 2P1/2 state, which decays back to the ground state levels with a branching ratio of 2/3 to me= -1/2 and 1/3 to me= +1/2. A strong holding eld (~1 kGauss) denes the axis of polarization and helps to overcome the radiation trapping effect in order to reach high optical pumping efciency. The spin is transferred to the H/D atoms through spin exchange collisions with a cross section of 7.4 ×10-15 cm2.

Gas Manifold: H2 or D2 pass through an MKS mass ow control, a pneumatic valve, and into the dissociator. The pressure is measured with a MKS baratron pressure guage.Dissociator: The resonator is cylindrical with an inner helical coil connected at one end to form a distributed LC circuit. While the lowest mode is at 36 MHz, we typically operate at 100 MHz and 35 W of power, less than 1% reected back. A 1 mm ID conductance limit before the spincell keeps the pressure in the dissociator at about 200 mTorr at a ow rate of 1 sccm.Spincell: 2” diameter pyrex sphere connected to a 1/2” diameter, 5 cm long transport tube, designed to minimize the number of wall bounces (640 in the spincell and 100 in the neck). They are coated with drilm (SC77 vapor followed by an afterwash) to minimize recombination and depo-larization effects on the wall. Target cell: an aluminum cylinder 12.5 mm diameter, 40 cm long. Polarized H/D enters a 6 mm aperature on the top, which makes the dwell time in the spincell 6 ms, much longer than the spin temperature equilibrium time constant. It is heated to 180°C.Holding Field: two 144 turn coils in a Helm-holtz-like conguration centered on the spincell. The maximum eld is 1 kGauss.Laser: a Spectra Physics model 3900S, 3 W Ti:Sapphire pumped with a 20 W argon ion laser. The beam is expanded to 1.5” diameter, and

passes through a λ/4 waveplate. A periscope with two polarization preserving mirrors is needed to illuminate the spincell, which is inset into the target chamber.Target Chamber: a six-way cross with 8” anges on the sides, and 10” anges on the top and bottom. It is pumped with a Varian 1000 l/s turbopump to 1.5 ×10-5 Torr with H owing at 1 sccm.

The laser driven target will be used in the “Precision Measurement of the Proton Charge Radius” experiment (RpEX), which has been conditionally approved by the Bates PAC. RpEX will measure the proton charge radius to an unprecedented 1% precision or better. The proton radius is a key input to high a precision test of QED (the hydrogen Lamb shift). It will also be a test of QCD as lattice calculations

become more accurate in the near future. The proposed experiment will combine the super ratio measurement of the spin-dependent elastic electron-proton scattering to determine the proton form factor ratio, and the relative unpolarized electron-proton scattering cross sec-tion. The projected results on the form factor ratio and rp are shown below.

In dissociator-only tests, we have reached up to 97% dis-sociation. The following graph shows the atomic fraction measured in test dissociators with different conductance limiters as a function of the ow rate. The black curve shows preliminary atomic fraction measurements from the full spincell/target cell setup.

This second graph shows the atomic fraction from the spincell and target cell at a ow of 1 sccm. First, the spincell was heated up to 180°C, (blue), and then the potassium ampoule was heated to 160°C (red). The atomic fraction dropped, but stayed near 60%.

The nal two plots show work in progress for polarization measurements. In each plot, the laser wavelength is scanned over the σ+ and σ- absorption lines at 770.105 and 770.115 nm. The blue points are the laser transmission through the spincell, and magenta points are the mass 1 signal. The measurements were caried out with defective polarization preserving mirrors (the circular polarization of the laser light after the periscope was only 88%).

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Hand et al. (1963)

Mainz, Saskatoon, OrsaySimon et al. (1980)

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