ELStiVIER Nuclear Physics A721 (2003) 1103c-1106~ www.elsevier.com/locate/npe

WITCH: Testing the Standard Model using a p-recoil Spectrometer with a Trapped Ion Cloud as Source

A. Lindrotha*, M. Beck”, B. Delaurk”, V. Yu. Kozlova, N. Severijns”, F. Amesb, D. Beck”+, V. V. Golovko”, I. Kraeva, T. Phalet”, and S. Versycka

“Katholieke Universiteit Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium

bLMU, Miinchen, Germany

The combination of a zero thickness source of p radioactivity, consisting of an ion- cloud in a Penning trap, with a retardation spectrometer has for the first time allowed the construction of a set-up that can measure the /3 recoil spectrum of any nucleus. This opens a number of severely hampered areas of study, such as lepton angular correlations and charge state distributions after P-decay. The former allows extraction of Standard Model violating S and T contributions to the weak interaction. An overview of the set-up is given in this presentation.

1. Testing the Standard Model

The fundamental coupling constants of the electro-weak interaction have presently been determined only with a precision that leaves much room for effects beyond the standard model, such as scalar contributions due to e.g. leptoquark or Higgs boson exchange [l]. For scalar (S) and tensor (T) interaction the present experimental limits are of the order of 10% of the V and A coupling constants, where the present limit for S contributions is from recoil spectrum measurements of the e-u angular correlation in P-decay [2]. Only very rare cases have allowed recoil studies of S and T contributions in P-decay (and p- recoil studies in general), and the first aim of WITCH (Weak Interaction Trap for Charged Particles) is to search for the presence of scalar currents, or improve the limits on their contribution, by measuring the e-B angular correlation, i.e. the aF coefficient, in pure Fermi decays. For an angle 8 between Y and e, the differential decay rate reads:

dw - = constunt ’ (1 + uFzcos8) dR

(1)

, where 6,s = 2 and ci = $$. In the Standard Model V-A description aF=l whereas S contributions would change the value of aF to be <l. The beta and neutrino emitted in p-

*Email: Axel.Lindroth@fys.kuleuven.ac.be ‘Present address: GSI, Darmstadt, Germany

0375-9474/03/$ - see front matter 0 2003 Elsevier Science B.V All rights reserved. doi:lO.l016/S0375-9474(03)01295-8

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decay have opposite chirality for V interaction, and the same chirality for S interaction. In superallowed O+ + Of Fermi decays where the lepton spins must couple to zero the leptons will be emitted preferentially into the same (opposite) direction for V(S) interaction. This leads to an on average larger recoil energy for V than for S interactions, and S contributions will thus affect the spectrum shape.

It is necessary to use the recoil method for the measurement of aF since the neutrino can not be detected easily enough. But recoil experiments have long been hampered by the fact that the p-emitter is usually embedded in a solid matrix, which leads to changes in the spectra due to energy losses. Recoiling daughter nuclei will most times be stopped already in the source due to their low energy, E, of usually E<lkeV.

By constructing a novel type of set-up [3] that can measure precisely the P-recoil spec- trum of any element, it is possible to choose freely among the best transitions available. Pure transitions in 26mA1 and 46V, and also 35Ar (which has a well-known aF from log ft measurements) are primary candidates. The problem of resolution, and even stopping, in solid elements is in WITCH circumvented by the use of a cylindrical Penning trap to store the radioactive ions. There they constitute a low-density ion cloud of basically zero thickness.

2. The WITCH Set-Up

The set-up [4] has to be vertical due to limited space in the ISOLDE hall at CERN. Total height is around 7m. The ions are produced and mass separated by ISOLDE, CERN where an extraordinary range of nuclides is available. REXTRAP traps and cools the beam, after which it is delivered in bunches to WITCH. These ion bunches of 6OkeV are decelerated in steps, one of which is a pulsed drift tube used to avoid a high voltage cage. Some fine retardations lead finally to injection with a suitable energy into a first Penning trap, the cooler trap. Here the ions are cooled to room temperature, and subsequently ejected into a second trap, the decay trap, where half of those which P-decay spiral into the spectrometer.

The spectrometer (see Fig. l), which bears some resemblance to the Mainz and Troitsk neutrino mass spectrometers, is based on the combination of two principles: 1) an inverse magnetic mirror to convert radial energy of recoiling ions into axial energy, which gives an acceptance approaching 21r, and 2) a retardation voltage that allows only recoils with sufficient energy to pass. Two super-conducting magnets, producing fields of 9T and O.lT, respectively, are used to create a homogenous magnetic field for the traps and a magnetic field gradient for the inverse magnetic mirror. They are housed in a common cryostat. The whole magnet system was delivered by Oxford Instruments, and tested at CERN during September 2002.

The two traps have been selected to be of Penning type since no restrictions are then imposed on the type of elements that can be trapped, and the cylindrical versions have an open end allowing free passage for the recoils. Penning traps should be hyperbolical in shape in order to allow the separation into three independent motions for the ions in the trap: cyclotron, magnetron and axial. Cylindrical Penning traps are equipped with correction electrodes to make the potential well sufficiently hyperbolic for the motion of the ions in the trap to be separated into the three independent motions.

A. Lindroth et al./Nuclear Physics A721 (2003) 1103c-1106~ 1105c

post-acceleration ion cloud B,= O.lT eptrodes

trap trap N~~---uv L 1 3 I

\ Analysiug plnne

--+?

---. \

detector

’ Eiuzel lens

Figure 1. Layout of magnet coils (solid) and electrodes (lines) for the combined retardation spectrometer and ion trap set-up WITCH.

The ions enter the cooler trap with an energy of -5OeV, where they are cooled by collisions with He buffer gas, and centered. This cooling is mass selective s rice it also i depends on a radio frequency (RF) quadrupole field applied to the central ring electrode, which is segmented. The first trap therefore removes isobaric impurities in addition to cooling and centering the ion cloud. The ions are ejected through a differential pumping barrier into the decay trap, where they are kept one to a few half-lives (a few seconds for the isotopes of interest). Most ions will decay during this time. The ion cloud in the decay trap will constitute the source of the experiments. Geometrically the two Penning traps are modified versions of the cooler trap for the ISOLTRAP high-precision mass measurement set-up in CERN. They are separated by a differential pumping barrier which keeps the He buffer gas away from the decay trap. A pressure ratio of a few hundred can be upheld across this barrier, and therefore one can achieve N lo-” mbar or better in the decay trap while still having enough buffer gas (i.e. N lop5 to 1O-3 mbar) in the cooler trap.

In the B-field a fraction of 1 - & = 98.9% of the radial energy of the recoil ions is converted into axial energy. This is of course paramount as it is then enough to probe the axial energy of the ions in order to reconstruct the recoil spectrum. A retardation voltage of up to a few hundred volts, corresponding to the recoil energies, is applied for this purpose. It is applied in steps of increasing radius in order that the electric field lines be parallel with the B-field lines. This ensures that electric retardation is focused only on the axial energy. After the retardation, the ions are accelerated to NlOkeV for two reasons: 1) to get off the B-field lines non-adiabatically, and 2) to reach energies where the multi channel plate detectors (MCPs) are efficient and have a flat efficiency versus energy curve. There is an Einzel lens for focusing onto the MCP. Simply varying the retardation voltage and counting in the MCP gives the spectrum. It will, due to the retardation principle, be an integral spectrum. Measurements will be done in the upper

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part of it, where the effect of S contributions is stronger and some problems in the lower part (see Fig. 2) can be avoided.

Limiting factors for the attainable pre- cision are the number of ions that can be stored in the traps (limited by space-charge effects), the fact that half of the decaying ions are lost because they have a momen- tum component in the backward direction, and that a fraction of the ions will not de- cay within the time they are kept in the de- cay trap and some will decay before reach- ing it. Under reasonable assumptions 10’ counts in the differential recoil spectrum is achievable, which leads to a sensitivity of &=0.50/o (la). This is at the forefront of Ghat is possible at present. Naturally, the aim is to improve on this after the principle has been seen to work. Test experiments will be run in 2003, and quantitative ex- periments in 2004.

Figure 2: Calculated differential recoil en- ergy spectrum. Different ionization states are indicated, as well as the electron capture peaks.

3. Other Experiments possible with WITCH

WITCH opens for the first time a general way for recoil spectroscopy in nuclear p- decay. A number of areas of research will benefit from this breakthrough. In addition to S interaction, probing for T interaction or F/GT ratios will be possible by measuring the “a” coefficient. From the positions of the EC peaks Q values can be inferred with a precision of few keV, which is nearly as good as ISOLTRAP. The, at present poorly known, charge state distributions following ,&decay can be measured. EC/P+ ratios and, in future upgrades of the set-up, excited states and polarized nuclei could be studied. In addition, a combination of WITCH with advanced techniques in nuclear spectroscopy (see e.g. [5]) would allow development of in-trap spectroscopy.

By combining a trap structure for the radioactive source with a retardation spectro- meter, which uses an inverse magnetic mirror principle for converting radial energy into axial, the step is taken from measuring the p recoil energy spectrum for the very rare cases amenable to study previously - a step to a general /3 recoil spectrometer!

REFERENCES

1. P. Herczeg, Prog. Part. Nucl. Phys. 46 (2001) 413. 2. E. G. Adelberger et. al., Phys. Rev. Lett. 83 (1999) 1299. 3. N. Severijns et. al., Hyp. Int. 129 (2000) 223. 4. M. Beck et. al., submitted to Nucl. Instrum. Methods Phys. Res. 5. A. Lindroth et. al., Phys. Rev. Lett. 82 (1999) 4783.