EPR Frequently Asked Questions
Xiaochao Zheng, email@example.com
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
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
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
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
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.