Atomic hydrogen beam


The ASACUSA collaboration has built a beam of cold atomic hydrogen with the initial aim to test the components of the antihydrogen hyperfine spectroscopy line. The setup can be also used to set limits to some coefficients of the non-minimal Standard Model Extension (ref. 1).

Purpose
Atomic Hydrogen Beam The reliable operation of the antihydrogen hyperfine spectroscopy apparatus is a basic prerequisite for the proposed CPT test and it requires a careful characterization of every component of the experiment, which can be done with hydrogen (with the obvious exception of the annihilation detector).  The spin–flip cavity and the superconducting sextupole magnet can be tested just as well with conventional atomic hydrogen. Therefore, a source of cold and polarized atomic hydrogen had been constructed and tested at SMI. It was shipped to CERN in late 2013 and has successfully been operated since 2014. This project resulted in a detailed characterization of the spin–flip cavity and the superconducting sextupole magnet, as well as a high precision verification (to a few parts per billion) of the apparatus and the measurement principle (ref. 2).

Principle
The hyperfine transition frequency of antihydrogen (hydrogen) will be determined by a  magnetic resonance measurement. This technique was invented by I. I. Rabi and improved by N. Ramsey (both Nobel prize winners) and requires the following four main components: (i) a beam of spin polarized particles, (ii) an oscillating magnetic field to induce spin-flips, (iii) a magnetic field gradient for spatial separation of spin states, and (iv) a detector for monitoring the amount of beam in the selected spin state.  When the frequency of the driving magnetic is close to one of the spin flip transition frequencies the beam intensity reaching the  detector is modified. By varying the driving frequency a well-understood resonance curve can be recorded from which the transition frequency is extracted with high precision. A noteworthy property of the antihydrogen beam line, distinguishing it from conventional magnetic resonance experiments, is that it accepts a much stronger diverging beam to make use of the largest possible fraction of antihydrogen atoms.

Setup
Setup schematic
The atomic hydrogen setup uses a source of polarized atomic hydrogen and a hydrogen detector. The spin flip driving cavity and the spin state selecting magnetic field gradient of the antihydrogen experiment form a Rabi-type beam experiment, as described above. Firstly, molecular hydrogen is generated via electrolysis from water, then  gets dissociated in a microwave driven discharge plasma to form atomic hydrogen. From this plasma atoms effuse into the vacuum of the beam line through a PTFE tubing kept at cryogenic temperature. This cooling mechanism results in a beam temperature of 50K to 100K, which is the anticipated temperature of the antihydrogen beam emerging from the CUSP trap. At this point the beam is a mixture of all spin states. The polarization is achieved by passage through a doublet of permanent sextupole magnets, in which a given spin component is selected. Furthermore, a chopper modulates the beam (tuning fork chopper: 180Hz, 50% duty cycle). This allows for a lock-in amplification of the detector signal. Next, the beam passes the microwave cavity which is optimized for ~1.42 GHz, the hyperfine transition frequency of hydrogen. Then, the magnetic field gradients of the superconducting sextupole magnet follows and defocuses those hydrogen atoms with a modified spin state, while focusing those with unchanged spin states onto the detector. Electrons from a filament are then used to ionize the hydrogen atoms. The resulting protons are electrostatically extracted and guided through a quadrupole mass spectrometer, which removes all other ion species from the residual gas (since they possess a different charge-to-mass ratio). The protons impinge on a channeltron for amplification and single-event counting. 

Current Status
The atomic hydrogen source and the detector have been constructed, assembled, and tested at the SMI in Vienna. In October 2013 the setup was transported to CERN and upgraded with the permanent sextupole magnets to become a source of polarized atomic hydrogen. In early 2014 the spin flip driving cavity and superconducting sextupole magnet of the antihydrogen experiment were inserted between hydrogen source and detector. Detailed characterization of all components followed and in April 2014 the first hyperfine transitions could be observed with this setup by using the Earth’s magnetic field as a guiding field. A magnetic shielding was then placed around the cavity and a pair of Helmholtz coils produced a homogeneous magnetic guiding field. An extensive measurement program has been completed in this configuration by September 2014, resulting in a measurement of the σ1-transition (see fig. 1 in hyperfine spectroscopy) in hydrogen with an accuracy of 2.7 ppb (ref. 2). The setup was then disassembled and the spin flip cavity and superconducting sextupole magnet were installed in the ASACUSA antihydrogen experiment at the antiproton decelerator facility.

Using a spare cavity and sextupole magnets made at SMI from commercial magnets in Halbach configuration, a modified setup was created that is able to simultaneously measure both σ and π transitions. The magnetic shielding and Helmholtz coils for producing the external constant field had to be upgraded. With this new setup measurements are under way to investigate a possible dependence of the hyperfine transition frequencies on the orientation of the external magnetic field, which will constrain some coefficients of the non-minimal Standard Model Extension (ref. 1) that have never been determined experimentally.

References:
1. A. V. Kostelecky, A. J. Vargas, Phys. Rev.  D92 (2015) 056002
2. M. Diermaier et al. , Nature Communications 8 (2017)  15749


See also:
Hyperfine structure: from hydrogen to antihydrogen
(CERN Courier, Nov. 2017)
A hydrogen beam to characterize the ASACUSA hyperfine spectrometer
(Nucl. Inst. and Meth. in Physics Research, A 935 (2019) 110)
In-beam hyperfine spectroscopy of hydrogen and antihydrogen (poster)

MS/AN 18.2.20