The SLAC high-density gaseous polarized 3He target☆
Summary (3 min read)
Introduction
- A large-scale high-pressure gaseous 3He polarized target has been developed for use with a high-intensity polarized electron beam at the Stanford Linear Accelerator Center.
- This target was used successfully in an experiment to study the spin structure of the neutron.
- Thus the spin of the nucleus is mainly provided by the neutron, and so polarized 3He is a good approximation to a polarized neutron target, diluted by the presence of the protons but with only small corrections needed due to the polarization of the protons.
- Second, practical 3He polarized targets have been developed [3–10] using the technique of collisional spin-exchange with optically pumped alkali-metal vapor, typically rubidium.
TARGET DESCRIPTION
- Figure 1 shows a schematic overview of the SLAC target system.
- Not shown is a second set of coils that produced a magnetic field (and 3He polarization) direction transverse to the beam direction for part of the experiment.
- The laser shown in the figure represents one of five Ti:sapphire lasers, each pumped by a 20 W argon-ion laser and capable of delivering several watts of power at 795 nm, the wavelength of the D1 line of rubidium.
- The other chamber was about 3.7 cm diameter by 8 cm long, and was enclosed in a plastic oven connected to hot air supply and return tubes (not shown).
- The oven and the support structure for the NMR coils and target cell were constructed of plastic or other nonconducting material, to avoid disrupting the RF drive field during NMR measurements.
THE TWO-CHAMBER CELL
- One requirement of the target arrangement was that the optical pumping of the rubidium vapor take place away from the electron beam, since the rubidium would be depolarized through ionization by the intense beam.
- In principle, a single cylindrical chamber could be pumped in a location outside the experimental area, then installed in the beam line to replace another cell when the polarization of the latter has dropped too low.
- In the SLAC environment, a long access time would be required to secure the experimental area, break the beam-line vacuum, exchange the target cells, then restore the vacuum, and finally retune the accelerator.
- Also, the glass cell walls could darken from radiation damage and prevent repumping of the cell, the target polarization would be varying at all times, recovery time from an accidental loss of polarization could be very long, and the pressure (and hence stress) while pumping would be substantially higher for the same operating pressure in the cell.
- The “double cell” solves or minimizes these problems, and in principle one good cell of this design could operate for the whole experiment.
MAXIMIZING POLARIZATION
- The asymptotic 3He polarization is given by P3He = (γSE/γSE+Γ) 〈PRb〉, where γSE is the spin-exchange rate between the rubidium and the 3He, Γ is the 3He spin relaxation rate due to all other effects, and 〈PRb〉 is the average rubidium polarization.
- If adequate laser power reaches all parts of the pumping chamber, 〈PRb〉 can approach 100%.
- Since γSE is proportional to the rubidium number density, this term may be increased by increasing the oven temperature to vaporize more rubidium, but then more laser power is needed to maintain 〈PRb〉.
- Practical limitations are then reached due to the cost and complexity of more lasers, or to the temperature limits of the oven materials, or both.
- The remaining variable is then the spin relaxation rate Γ, which should be minimized.
RELAXATION EFFECTS
- The total 3He spin relaxation rate is the result of several effects, here approximately Γ = Γbulk +.
- At a pressure of about 10 atm, this limits the relaxation time to approximately 75 hours.
- This was not a major effect during much of the experiment, but caused a noticeable (several percent) reduction in target polarization at the highest beam currents (about 4 µA).
- The remaining terms are ones over which the target builders have some control.
- The last two effects listed are due to collisions of 3He atoms with paramagnetic impurities in the GAS mixture, and collisions with the cell WALLS.
CELL MANUFACTURE
- Figure 2 shows a diagram of the system used for preparing and filling target cells.
- A small amount of rubidium is then “chased” into the pumping chamber with a torch, and the target chamber is inclosed in a vacuum-walled enclosure through which liquid 4He is blown.
- Aluminosilicate glass has been found to highly suitable for this purpose, possibly due to its relatively low porosity to 3He.
- For the cells used in this experiment, commercial tubing (Corning 1720) was rinsed with nitric acid to remove possible surface contaminants, then reblown to the desired dimensions on a glass-working lathe, resulting in a very clean and microscopically smooth “fire-polished” surface.
- Cells constructed in this manner and filled as described above were measured to have net relaxation times up to 65 hours at room temperature with no incident electron beam, or nearly the bulk limit.
POLARIZATION MEASUREMENT
- The method chosen for measuring target polarization was the NMR technique called Adiabatic Fast Passage, or AFP.
- An adjustable circuit (“A-φ Box”) produces a signal used to cancel any direct pickup from the drive field.
- Repeated measurements show that very little polarization (< 0.1%) is lost in this procedure.
- The method is calibrated by replacing the 3He cell with a nearly identical cell filled with distilled water, then performing repeated measurements with the same apparatus at the same frequency.
- Figure 4(b) shows the average of 25 proton signals from a typical water measurement.
OPERATIONAL EXPERIENCE
- Figure 5 shows the measured 3He polarization as a function of time during the sixweek experimental run.
- (1) The first (and best) cell installed had to be removed when the plastic oven cracked and leaked due to prolonged exposure to high temperature while surrounded by vacuum.
- The arrangement was not capable of optical pumping in this orientation, so the polarization decayed until the longitudinal orientation was restored.
- For the transverse running, the main holding field was provided by a second set of Helmholtz coils, as mentioned earlier.
- This will increase the average rubidium polarization, or maintain it in the face of higher rubidium density.
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...With the notable exception of the early SLAC5 3He targets that used up to six Ti:Sapphire lasers (Johnson et al., 1995), these large, expensive and often unreliable sources were generally limited to 10 W of pumping light....
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Related Papers (5)
Frequently Asked Questions (17)
Q2. Why is the target chamber at a low temperature?
Due to the low thermal conductivity of glass, the target chamber remains at a low enough temperature (about 60˚C) that the rubidium vapor density there is negligible.
Q3. What is the laser shown in the figure?
The laser shown in the figure represents one of five Ti:sapphire lasers, each pumped by a 20 W argon-ion laser and capable of delivering several watts of power at 795 nm, the wavelength of the D1 line of rubidium.
Q4. What was the largest contribution for this experiment?
The largest contribution for this experiment was from the extraction of the proton signal from water measurements, with an estimated uncertainty of ±5.6%.
Q5. What was the polarization of the laser beam?
The quarter-wave plate in each laser beam line was adjusted to produce circularly polarized photons, and the mirrors shown were oriented so as to preserve the circular polarization.
Q6. What caused the beam intensity to increase?
(5) Near the end of the experiment the beam intensity was increased, leading to additionaldepolarization due to ionization by the electrons.
Q7. What is the disadvantage of this design?
The disadvantage of this design is that the effective spin-exchange time is increased, and the cell is more difficult to construct.
Q8. What is the principle of the design?
In principle, a single cylindrical chamber could be pumped in a location outside the experimental area, then installed in the beam line to replace another cell when the polarization of the latter has dropped too low.
Q9. What was the purpose of the oven and the target cell?
The oven and the support structure for the NMR coils and target cell were constructed of plastic or other nonconducting material, to avoid disrupting the RF drivefield during NMR measurements.
Q10. What is the evidence that the electron scattering rates dropped linearly with the 3He NMR?
The evidence is that the electron scattering rates dropped linearly with the 3He NMR signals during this episode, implying a decrease in 3He density rather than polarization.
Q11. What is the polarization of the rubidium vapor?
The asymptotic 3He polarization is given by P3He = (γSE/γSE+Γ) 〈PRb〉, where γSE is the spin-exchange rate between the rubidium and the 3He, Γ is the 3He spin relaxation rate due to all other effects, and 〈PRb〉 is the average rubidium polarization.
Q12. What is the polarization of the rubidium laser?
The large main Helmholtz coils have a diameter of about 1.5 m and provide a uniform magnetic field of about 20 G in the vicinity of the target cell during operation.
Q13. How long did the cell have to be filled?
Cells constructed in this manner and filled as described above were measured to have net relaxation times up to 65 hours at room temperature with no incident electron beam, or nearly the bulk limit.
Q14. What is the polarization of the 3He in the upper chamber?
In this design, the 3He is polarized in the upper (pumping) chamber and diffuses through the transfer tube to the lower (target) chamber with a time constant of about 10 min—small compared with the characteristic spin exchange and relaxation times.
Q15. What is the polarization of the proton in the target chamber?
The 3He density in the target chamber was determined from measurements made during cell filling, and from temperature measurements of the two chambers of the cell during operation, and contributes an estimated uncertainty of ±2.5%.
Q16. What is the polarization of the 3He laser?
The RF drive coils are about 50 cm in diameter and, together with the small pickup coils shown, were used for NMR measurements of the 3He polarization, as described later in this paper.
Q17. What was the effect of the beam currents on the target?
This was not a major effect during much of the experiment, but caused a noticeable (several percent) reduction in target polarization at the highest beam currents (about 4 µA).