3.2 Experimental apparatus
The most detailed desription of the RAL/Sussex/ILL nEDM spectrometer is available from Ref. [55, Chapt. 5], here we will focus only on the most relevant elements. The schematic of the apparatus is shown in Fig. 3.2.
Figure 3.2: RAL/Sussex/ILL nEDM spectrometer (from [54]).
3.2.1 UCN guides
The neutrons, coming from the ILL UCN turbine (described in Ref. [2]), are guided to the apparatus by a 5.9 m long horizontal tube (78 mm inner diameter) made of stainless steel, which has been electropolished and coated with a thin layer of58Ni/Mo.
The loss rate of the horizontal guide is approximately 10% per meter. At the switch pot, neutrons are directed upwards to the storage chamber via vertical guides; first a 25 cm long Ni/Mo coated copper section, then a 1.5 meter long beryllium coated glass tube, both of 68 mm inner diameter. Glass and copper tubes are used in order to avoid remanent magnetisation, typical for stainless steel tubes, which would depolarize
20 Chapter 3. The nEDM Experiment
UCN. Additionally, glass can be penetrated by the oscillating field produced by the spin flipper coil (see 3.2.3).
3.2.2 The storage chamber
The storage chamber consists of a hollow electrically insulating cylinder located be-tween a HV electrode (top) and a ground (bottom) electrode. The full volume of the trap is approximately 20 liters. At present the insulator is made of fused silica HSQ300. It has UV grade fused silica (UVFS) optical windows on either side, of 50 and 57 mm diameters, which roughly correponds to the divergence of the UV light beam, used for the 199Hg co-magnetometer.
The electrode surfaces are coated with DLC with a Fermi potential of about 225 neV. The cylinder fits into 15 mm deep grooves in both electrodes, which re-duces HV breakdown probability. Both UCN (78 mm diameter, DPS coated) and
199Hg (14.5 mm diameter, Teflon) shutters are recessed into the bottom electrode in order to minimize their influence on HV stability. The shutters are controlled by pistons powered by compressed air. The gas tight seal between the insulator and the electrodes (necessary for the 199Hg operation, see below) is made with two teflon o-rings located in the grooves.
An electric field of up to 180 kV (which translates to 15 kV/cm) is applied to the top electrode and its polarity is reversed after each series of measurement cycles.
High voltage is provided via the HV feedthrough and a cable, with a 1 MΩ resistor close to the feedthrough, connected to a HV generator.
It is essential that all the materials used in the vicinity of the chamber are not magnetic.
3.2.3 UCN polarization and detection
The polarizer foil (either a silicon wafer or an aluminum foil coated with a 200 nm layer of iron, magnetized with by permanent magnet positioned around the foil) is located between the two vertical guides, mentioned before. According to Eq. 2.6, the foil will have different potentials depending on the orientation of the neutron spin, VF ± |µn·B| = (204±120) neV. Neutrons with spin oriented in the direction of the field within the foil (called spin down) experience a Fermi potential of about 324 neV and are reflected (for Ekin. < 324 neV), while those with opposite spin orientation (called spin up) experience only 84 neV and most of them can pass the foil. Polarizations of ∼90% are typically achieved in this way.
Just above the magnetized foil, an adiabatic spin flipper is located. This spin flipper, which consists of a RF longitudinal coil in combination with the transverse, linearly decreasing fringe field of the polarizer magnet, is used normally at the end of the measurement cycle during emptying. The polarizing foil acts at that time as a spin analyzer; UCN which are in the spin up state pass through the polarizing foil, the remaining ones are unable to do so until the spin flipping coil, located above the foil, is turned on, reversing their spin orientation. During emptying the switch-pot provides a direct connection to the 3He, detector located ∼0.5 m below the foil.
3.2. Experimental apparatus 21
Figure 3.3: Location of the fluxgate meter: common view of the storage chamber (left) and its location in the vacuum tank (see also Fig. 3.2). The fluxgate was fixed on the top electrode and aligned with the main axes. Pictures from [56].
Typically, spin up neutrons are counted for 8 seconds, then the spin flipper is turned on and spin down neutrons can pass the foil for the next 20 seconds. At the end, the spin flipper is turned off and spin up neutrons are detected again, this time for 12 seconds. The counting periods are adjusted such that for unpolarized neutrons there is almost no asymmetry between spin up and spin down counts. However, because of storage effects during emptying some small residual asymmetry might still appear.
Since the measurement of dn is based on a relative change of this asymmetry upon E-field reversal, this is not a problem.
3.2.4 The magnetic field
Equation 3.1 shows that it is necessary to control very precisely the magnetic field in the storage volume. In order to keep the systematical uncertainties on a level of 10−26ecm a certain degree of homogeneity (∼10−3) and temporal stability (∼10−5) is required. The four layer µ-metal shield significantly suppresses the influence of the ambient fields, nevertheless, strong external fields still can penetrate the inside to some extent. It has several holes necessary, e.g. for the HV feedthrough, UCN guides, mercury prepolarization chamber, vacuum system etc., which unfortunately affect the field inside the vacuum tank. The ends of the shield can be removed to gain access to the storage chamber.
We have recently re-measured the axial and longitudinal shielding factors. A 1.4 m diameter external coil was used to generate a magnetic field of known magnitude (of
22 Chapter 3. The nEDM Experiment
order 10µT at a distance of one meter from the coil center) and the field change inside the chamber was measured with fluxgate meters1, located on the top electrode (see Fig. 3.3). The summary given in the Tab. 3.1 compares the measured values with the ones calculated employing both the “accurate” and the approximated Dubbers formula [57]. The magnetic permeability of the µ-metal was assumed as 20000 for the calculation.
Axis Exact calculation Approx. calculation Measurement
X 24023 22000 21500
Y 3550 3460 1935
Z 24023 22000 6500
Table 3.1: Comparison of calculated and measured shielding factors for the magnetic shield of the nEDM experiment, where “X” corresponds to the shield symmetry axis.
The most critical direction is “Z”, because vertically oriented fields would directly affect the spin precession frequency.
The shielding factor of the nEDM magnetic shield could be then understood with a value of µ∼20000 (or better, and stronger influence of the holes) [58].
The shield properties can be negatively affected by mechanical stresses and ther-mal expansion, therefore each time after it is disturbed, e.g. by opening, a demagneti-zation procedure is conducted. A slowly oscillating current is sent through dedicated coils wrapped around the shields. The remanent magnetization is cycled over the µ-metal hysteresis loop. Over about 20 minutes the oscillation amplitude is gradually reduced to zero, and so is the magnetization of the shields.
Batch-by-batch magnetic field variations observed in the experiment of the order of 10 pT are common and sudden jumps about one order of magnitude higher would occur several times per day [59].
The coil, which generates the 1µT main static guiding fieldB0, is wound directly onto the vacuum vessel (see Fig. 3.2), with a constant number of turns per unit vertical distance. The field is aligned in the vertical direction and is driven by a high-stability current source.
3.2.5 The
199Hg magnetometer system
As theB0field stability is the central assumption, it is monitored during the measure-ment with a Hg vapor co-magnetometer. The precession frequency of the neutrons and 199Hg atoms is given by respective formulas
ωn=−γnBn, ω199Hg=−γ199HgB199Hg, (3.4) where gammas are the gyromagnetic ratios and B is the strength of the experienced magnetic field. The magnetic field experienced by the neutrons can be deduced from
Bn =−ωn γn
=−Rω199Hg Rγ199Hg = R
RB199Hg, (3.5)
1Mag-03MC500 and Mag-03MCL70 from Bartington