Development of a 3He magnetometer for a neutron electric dipole moment experiment
© Kraft et al.; licensee Springer on behalf of EPJ. 2014
Received: 3 July 2014
Accepted: 30 July 2014
Published: 1 October 2014
We have developed a highly sensitive 3He magnetometer for the accurate measurement of the magnetic field in an experiment searching for an electric dipole moment of the neutron. By measuring the Larmor frequency of nuclear spin polarized 3He atoms a sensitivity on the femto-Tesla scale can be achieved. A 3He/Cs-test facility was established at the Institute of Physics of the Johannes Gutenberg University in Mainz to investigate the readout of 3He free induction decay with a lamp-pumped Cs magnetometer. For this we designed and built an ultra-compact and transportable polarizer unit which polarizes 3He gas up to 55% by metastability exchange optical pumping. The polarized 3He was successfully transfered from the polarizer into a glass cell mounted in a magnetic shield and the 3He free induction decay was detected by a lamp-pumped Cs magnetometer.
07.55.Ge Magnetometers for magnetic field measurements; 13.40 Electric and magnetic moments; 14.20 Protons and neutrons
In 1950, Purcell and Ramsey  were the first to point out that the existence of an electric dipole moment (EDM) in an elementary particle with spin would violate the parity symmetry P. Although there were strong arguments favoring the view that elementary particles and their interactions conserve parity, Ramsey and Purcell called for an experimental search for EDMs. Shortly after the discovery of P violation in 1957 - in weak interactions, Smith, Purcell, and Ramsey  reported the first (null) result of an experimental search for a neutron EDM. Besides P, an EDM also violates time reversal invariance, T, and the discovery of CP-violation in the neutral K meson’s decay  in 1964 (being equivalent to T-violation when assuming CPT conservation) removed objections against the possible existence of an EDM in elementary particles. However, besides the recent observation of CP-violation in the B meson system ,, no other experimental evidence for CP- or T-violation has been revealed in elementary particle physics experiments nor in experiments on atoms or molecules up to date. Among all elementary particles, neutrons provide the best possibilities for the EDM search, since they are electrically neutral, may have a long-lived spin coherence, and can be stored for a long time in an environment which can hold a strong electric field.
The current upper limit of the neutron electric dipole moment (nEDM) , dn ≤ 2.9·10−26 e·cm (90% c.l.), represents one of the most precise measurements in physics and continues to challenge our understanding of fundamental physics, in particular the so-termed "‘Strong CP-Problem"’ and "‘SUSY CP-Problem"’, see, e.g. ,. Today, various efforts to improve the sensitivity to an nEDM are underway -. The nEDM collaborationa at the Paul Scherrer Institute (PSI) in Villigen, Switzerland is currently developing a next generation EDM setup which will use ultracold neutrons (UCN) delivered by a new spallation-based UCN source with a solid deuterium moderator/converter combination . The nEDM experiment at PSI uses the conventional approach of storing UCN in a trap at room temperature. Additional improvements of the experiment are aimed at a sensitivity gain of more than one order of magnitude. The gain in measurement sensitivity, sets new challenges to the monitoring of systematic effects, in particular effects associated with magnetic field changes that are correlated with the switching of the electric field and which may mimic a false nEDM signal.
In this paper, we describe the technical realization of a 3He magnetometer system that shall be implemented into the PSI n2EDM experiment.
3He magnetometer for the n2EDM experiment
Precise knowledge of the magnetic field and its vertical gradient inside the neutron precession chamber during Ramsey cycles is of crucial importance for controlling several systematic effects. One approach described here, is to infer the magnetic field amplitude from the Larmor precession frequency of nuclear spin polarized 3He, detected by its free induction decay (FID) .
The 3He magnetometer will be placed in the electric field free region near the UCN storage chamber to avoid geometrical phase dependent systematic effects on the 3He nuclei proper . Since the helium magnetometer is based on the free precession of nuclear spins, it is not prone to many systematic frequency shifts that are inherent to phase-locked optically pumped alkali vapour magnetometers and additionally gains in overall sensitivity due to the long 3He coherence times. The proposed 3He sandwich configuration, Figure 1, enables the control of vertical gradients of the magnetic holding field with high precision, whose knowledge and control is of great importance for tracing tiny geometrical phase effects in the nEDM spectrometer.
A magnetometric sensitivity in the femto-Tesla range has been demonstrated using SQUID (superconducting quantum interference device) magnetometers for the readout of the precessing 3He spins . However, the required cryogenic infrastructure is a severe obstacle for operating these magnetometers under the experimental conditions of an EDM experiment (vacuum, high voltage, stabilized temperature, exclusive use of nonmagnetic materials). As shown for the first time by Cohen-Tannoudji et al.  in 1967 optically pumped alkali (Rb) magnetometers are a promising alternative for the readout of the 3He precession signal. Based on the expertise with optically pumped Cs magnetometers (CsOPMs) gained in the ongoing phase of the PSI-nEDM experiment, we plan to deploy a CsOPM-based, rather than a SQUID-based detection of the 3He FID signal. The CsOPM are developed by the Fribourg Atomic Physics group (FRAP) -, member of the nEDM collaboration at PSI. The polarizer unit necessary for spin polarization of the 3He gas was built at the Johannes Gutenberg University (JoGu), Mainz, and is described in detail below.
During nEDM measurement, 3He at a pressure of 1 mbar will be polarized in an ultra-compact and transportable polarizer unit (UCPU) outside the n2EDM chamber in a field of 1 Gauss. The gas is then compressed to a pressure of ≈ 100 mbar and stored in low-relaxation glass cell. The whole UCPU is enclosed in a mu-metal cylinder to provide shielding from environmental fields and to allow generation of a homogeneous magnetic field by an internal cylindrical coil. Upon request, the compressed and polarized 3He in the storage cell will be transferred into the magnetometer vessels through a polytetrafluoroethylene (PTFE) tube surrounded by coils producing a suitable magnetic holding and guiding field on the order of 1 to 200 μ T.
The magnetometer vessels will be two flat cylindrical glass cells mounted inside the n2EDM vacuum tank, above and below (top, bottom) the neutron spin precession chamber, respectively (Figure 1). Since both magnetometer cells are traversed by almost the same magnetic flux as the UCN chambers, the average value of frequency measurements ωHe=(ωt,He+ωb,He)/2 yields a best guess for the neutron spin precession frequency ωn=(γn/γHe)ωHe, while the frequency difference determines the magnetic field gradient ∂|B|/∂ z=(ωt,He−ωb,He)/(γHe·Δ z) to a high precision. Here, Δ z is the distance between the centers of gravity of the upper and lower magnetometer vessel, ωt,He and ωb,He are the free spin precession frequencies of the 3He in the top and bottom magnetometer vessels, respectively, and γHe is the 3He gyromagnetic ratio.
The 3He FID is started by applying a π/2 spin-flip pulse. The long coherence time of the 3He FID signal, which strongly depends on the absolute magnetic field gradients (∼ 20 pT/cm), the magnetometer cell material, the size of the magnetometer vessels as well as the 3He pressure (p∼mbar), reaches the order of one hour under the typical operating conditions of the EDM apparatus. Assuming a typical Ramsey cycle time for the ultracold neutrons of ≈ 200 s, it is thus possible to run 2–3 Ramey cycles with a single 3He filling without significant decay of the 3He FID signal amplitude. After these cycles the partly relaxed 3He will be pumped out, eventually recovered  and replaced by freshly polarized gas that has been prepared by the UCPU during the Ramsey cycles. This procedure will require the UCPU to deliver freshly polarized 3He gas every 10 to 15 min in order to ensure a high signal to noise ratio (SNR) of the recorded spin precession signal. A quasi-continuous operation of the 3He magnetometer will thus be possible.
Results and discussion
3He/Cs magnetometer test facility
A dedicated test facility was installed at the JoGu in order to construct and test the proposed 3He/Cs magnetometer system. As main part of this facility, a cylindrical four layer mu-metal shield (MS)  with magnetic shielding factors of 100 (longitudinal) and 1000 (transverse) was adapted to the requirements imposed by the 3He FID studies. In order to lower existing magnetic field gradients and to record the 3He FID signal, the innermost mu-metal cylinder (610 mm diameter, 1300 mm length) was equipped with a magnetic coil system (solenoid and cosine-theta coil) providing magnetic fields parallel and perpendicular to the cylinder axis. The coils were wound on a double-walled cardboard cylinder which was then covered with five layers of Metglas . With these arrangements, magnetic field gradients of 150 to 350 pT/cm could be achieved in the longitudinal direction. For the transverse field gradients, values in range of 350 to 450 pT/cm were determined. Within this magnetic environment two basic types of experiments have been performed:
- FID detection of in-situ polarized 3He in a small cell, and
- FID detection of externally polarized and transferred 3He.
For the field (≈ 1 μ T) applied in the present study, ωCs/2π is ≈ 3.5 kHz. When the solenoid coil is powered and a discharge is ignited inside the cell, the 3He pump laser produces nuclear spin polarization along the direction of the solenoid axis. The degree of polarization reaches approximately 65% after two minutes of optical pumping. There are two different ways to initiate the 3He FID after the discharge and the laser have been switched off:
- A pulse of resonant rf radiation can be applied with the cosine-theta coil to rotate the 3He spin polarization by 90° (π/2 spin-flip pulse).
For the measurement presented in Figure 4, Eq. 5 yields bHe = 8.5 pT which represents a SNR of about 34 in a bandwidth of 1 Hz. We inferred the -time from a measurement of the time dependent decay of the amplitude AHe. For different measurements in both the Bsol and Bcos field configurations spin coherence times of up to one hour were obtained. The feasibility of the new test setup for further investigations of the functionality of an ultra-compact 3He polarizer unit (UCPU) is successfully demonstrated.
Ultra compact polarizer unit
As mentioned above the main component of the future 3He magnetometer system, the 3He polarizer unit (UCPU) should be able to deliver periodically i.e., every 10-15 min, ∼16 mbar·l of spin-polarized 3He gas, corresponding to a 3He pressure of about 1 mbar in the two magnetometer vessels that have a total volume of 16 l. The UCPU is an ultra-compact transportable 3He polarizer following several generations of polarizer facilities which were developed at JoGu for medical application respectively fundamental research -. It has been specifically designed to fulfill the needs of the planned n2EDM experiment at PSI.
Polarization maintaining transfer
With a polarization P of 52%, a pressure p of 30 mbar and a volume V of 0.09 l one obtains m=3.66·10−7 Am2. Assuming that at t=0 the magnetic moments are oriented in the -direction and knowing that the centers of the 3He cell and the CsOPM cell are placed in the plane of the MS coil system on the 45° cone (Figure 3) at a distance r of 70(2)mm the expected 3He magnetic field seen by the CsOPM is bHe=160(14) pT. The difference between the prediction, assuming a lossless transfer, and the measured value is below 10%. From this we expect the transfer losses to be 2.0(9)%.
A test facility for the recording of 3He FID by a lamp-pumped CsOPM was installed at University of Mainz. A compact, transportable 3He polarizer was built and its functionality demonstrated. It was shown that the device is capable of delivering the required amounts of polarized gas for the n2EDM experiment with 52% of spin polarization during cyclic operation. For the first time the transfer of polarized 3He gas from a cyclically working polarizer into a multilayer mu-metal shield, even through a transfer line having two 90° bends, with negligible depolarization was successfully demonstrated. Thus, together with 3He magnetometers made of flat cylindrical cells , all essential 3He magnetometer parts (see Figure 1) were built and tested. Further improvements of the magnetometer largely depend on the magnetic field conditions in the next generation nEDM spectrometer (longer 3He transverse relaxation times, lower magnetic field noise, higher SNR in the 3He precession detection). Besides this the construction of a recovery and recycling system for used 3He gas will help to drive the whole magnetometer setup in a more economical way . A detailed analysis of the achievable sensitivity and possible systematic effects in the n2EDM spectrometer with a combined 3He/Cs-magnetometer is ongoing and will be published elsewhere.
a list of n2EDM members, see also http://nedm.web.psi.ch.
b C8: 2 3S1, F=1/2 → 2 3P0, F=1/2, λ=1083,06 nm.
The described work benefited from the excellent support of the mechanical workshops of the Institute of Physics and the Institute of Nuclear Chemistry of the University of Mainz, especially L. Funke and glassblower R. Jera. We also thank Yu. V. Borisov, Konstantinov Nuclear Physics Institute, St. Petersburg, who did some preliminary studies including prototyping of the flat magnetometer vessels and 3He polarimetry. The research was enabled by the loan of the mu-metal test shield by K. Kirch and B. Lauss, PSI, Switzerland. This work was supported by the DFG under the contract number HE 2308/10-1. We wish to thank M. J. Kraft, Wiesbaden, Germany providing support in the creation of the illustrations.
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