EPR Frequently Asked Questions

EPR Frequently Asked Questions

Xiaochao Zheng, xiaochao@jlab.org

For the polarized 3He target group @ Jlab Hall-A

June 6, 2001

What is EPR polarimetry?
EPR measurement is an alternative way to measure the polarization of polarized 3He target. It can be used either as a direct on-line monitoring of target polarization, or as a calibration for NMR measurement, in addition to the water calibration.
What is the principle of EPR measurement?
EPR polarimetry measures the frequency shift of Rb Zeeman resonance caused by polarized 3He inside pumping chamber. The absolute frequency of Rb Zeeman resonance is determined by both the holding field (25 Gauss) and a small field caused by the polarization of 3He (about 0.1 Gauss). Because the holding field fluctuates at a level of 10E-3, it is necessary to reverse the direction of 3He polarization (usually this is done by AFP). By measuring the frequency difference between these two polarization directions, the instability of holding field is cancelled so one is able to extract 3He polarization.
How do we measure Rb Zeeman resonance frequency?
To detect the EPR resonance, we rely on the fact that during optical pumping the polarization of Rb vapor is very high. Most of the Rb atoms are in the F=3, M=-3 state (or M=+3 state for oppositely polarized light). Laser light can penetrate quite far into the cell because most Rb atoms are in the state which cannot absorb circularly polarized photons. Among the atoms that do absorb photons and are excited to P state, most of them will be radiationlessly quenched to ground state by Nitrogen filled into the cell. A small fraction (3~5%) will decay by emitting fluorescence photons at either D1 (wavelength 795 nm) or D2 (wavelength 785 nm) line. Lasers scattered by the cell are tuned to D1 transition frequency, so the D1 light which one could measure is dominated by the scattered laser instead of fluorescence. Only D2 light is directly proportional to fluorescence and is what should be measured during EPR measurement.
The intensity of fluorescence is proportional to the rate of photon absorption within the pumping chamber. If we apply an RF field at the EPR frequency corresponding to M=-3->-2 transition, it will equalize the population of these two states. The number of atoms in the M=-2 state which are capable of absorbing laser light will increase and therefore, the intensity of fluorescence will increase. By monitoring the intensity of fluorescence as a function of RF frequency we can detect the EPR resonance.
What is the most difficult thing of EPR measurement?
The system built for EPR measurement consists two parts -- lockin unit and spin-flip unit. The lockin unit applies a RF field to the cell, lockin into the D2 fluorescence excited by this RF field, and track its frequency. RF field is necessary for measuring D2 light, but at the same time it causes a field gradient which will depolarize the target. Hence the most difficult part is to optimize the whole system so the D2 signal induced by RF field is strong enough to perform the measurement, at the same time the polarization loss caused by this RF field is minimized. To reduce the polarization loss, the crucial thing is to minimize the time of activation RF field, which means, to do the measurement as quick as possible.
However, this is not easy since the D2 fluorence is usually very weak. A PIN photodiode is used to measured this fluorescence. A D2 filter is placed right before photodiode to block D1 light. Though most of D1 light can be blocked by the D2 filter, it still dominates the photodiode signal. For example, typical output from photodiode is about 150 mV, within which only 30 microVolts is given by D2 light. For an experienced operator, it takes less than 20 seconds to find the D2 signal, and 2 minutes to perform the AFP flip measurement. However, for those who are not familiar with the system, it might take one hour or longer to set the circuit properly and hence track the fluorence signal. Applying 10 minutes' RF field will cause a 2% polarization loss, which is already comparable to the loss cause by AFP flip and should be avoided.
What will happen if EPR goes wrong?
Good question. During an EPR measurement, once the 3He spin is reversed, Rb resonance frequency will jump by about 50 KHz. If the electronics is not optimized and lose the lockin, the spectrum shown on LabView screen will look really wierd. However, if one keeps calm and let the program to finish, then we only lose about 2% polarization (relative) due to the AFP spin flip. But, if the program is stopped in the wrong state, which means, 3He spins are reversed to the opposite direction, but are not flipped back, then they will stay in the reversed state, and will depolarize dramatically. For example, the polarization will drop from 40% to 20% in a few hours. For an experienced EPR operator, it is possible to find the resonance again, re-start the Labview program and flip 3He's back, but it is difficult and 3He's are very unstable. A small change of the electronics - for example, disconnect RF capacitor (which is the next step of EPR operation procedure) - could push 3He's to the wrong state again. Such depolarization had happened several times during tests in target lab. In the worst case, we do not even know where they are - "Mirror, mirror on the wall, what is the helium state in the cell?" "Heliums, come back, please."
What is EPR optics?
In principle the photodiode can be put anywhere close to the cell since a small amount of light is enough for the measurement. That is what we did during tests in target lab. However, during the real runs down in the hall, the photodiode will be damaged by radiation in a few hours if it is positioned close to the cell. During E94010 and E95001 in 1998, EPR was only performed during maintainance day, when people can access to the hall and put photodiode on top of the oven. For E99117 and E97103, we construct an optics system which can focus the fluorence light and lead it to seven meters away from the target pivot, where there is enough space to put shielding for the photodiode. The EPR optics for E99117/E97103 consists (1) two lenses and two mirrors inside the target enclosure, which focus the fluorence light and lead it out of the target, and (2) nine lenses, related supports and light tubes which lead fluorence to photodiode, and protect people from the light (lasers). The difficult part of EPR optics is that the divergence and distribution of the fluorence light emitted by the cell are irregular, depending on laser configuration, RF field etc.. During alignment we can only use D1 light (which is visible through an infrared scope) to adjust the optics. How much D2 light will eventually reach the photodiode is unknown.
Why cannot we do EPR measurement for transverse lasers?
The current EPR optics is aligned using longitudial lasers. For transverse lasers, nothing reaches photodiode. In the future it is possible to re-align EPR optics using transverse lasers so one can perform EPR in transverse mode. This is very exciting since the other polarimetry -NMR- only works in the longitudinal mode. Although tests have been done to study the polarization loss caused by the procedure of "rotate field"->do NMR->"rotate field back" and the result shows that the loss is trivial, it would be nice if we can run polarimetry directly at transverse mode, which can only be performed through EPR.
Do you have any plots for typical EPR measurement?
Yes. We have some plots for Ouch from tests in target lab. Though the system has been changed and optimized several times in the last 4 months, these plots are typical for EPR measurement and the off-line analysis.