HREPHAH 0H3HKA, 2004, moM 67, № 6, c. 1140-1145



©2004 V. Yu. Kozlov, M. Beck*. F.Ames1), D. Beck2), S. Coeck, P. Delahaye1), B. Delaure, V. V. Golovko, A. Lindroth, I. S. Kraev, T. Phalet, N. Severijns, S. Versyck

(and the ISOLDE and NIPNET Collaborations)

IKS, K.U. Leuven, Belgium Received November 4, 2003

The WITCH experiment (Weak Interaction Trap for CHarged particles) is starting measurements at the ISOLDE facility at CERN at present. It has been set up to measure the energy spectrum of the recoiling daughter ions after nuclear beta decay for precision tests of the Standard Model of the weak interaction. However, many other topics of interest are accessible. In this article the possibilities of recoil spectroscopy with the WITCH experiment are discussed as well as the principle of the setup and its present situation.


The Standard Model (SM) of the electroweak interaction is very successful in describing the interaction both qualitatively and quantitatively. Nevertheless, it contains many free parameters and ad-hoc assumptions. One of these is that from the five possible types of weak interactions — vector (V), axial-vector (A), scalar (S), tensor (T), and pseudo-scalar interaction (P) — just V and A interactions are present at a fundamental level. Together with maximal parity violation this leads to the well-known V — A structure of the weak interaction.

The effective Hamiltonian for [ decay, including all possible interactions, is given by

Hß =

(pn) {e{Cs + CS75) V + (1)

+ {pYßn) (ê-Yß {Cy + C'y 15) V +

+ \ (P<r\ßn) [ëa^ (Gr + C'Tlh) v) -

- {PYßY5n) (eiß 15 {Ca + C'A75) v) +

+ (PY5n) (ei5 (Cp + C'p75) V

+ h.c.

''CERN, Geneva, Switzerland.

2)GSI-Darmstadt, Darmstadt, Germany. E-mail: Valentin.Kozlov@fys.kuleuven.ac.be,


of the limits for the S and T coupling constants (CS, CS and Ct , CT) in the charged current sector are rather weak [1 —6], i.e., of the order of 10% of the coupling constants of the V and A interactions (see the table). This was deduced mostly from measurements of the [-neutrino angular correlation in nuclear [ decay. For unpolarized nuclei this correlation can be written as [7]

p • q -&

fa \ 11 EE - . v „ _ rm, u){Ovp) = 1 +

EEV rm.

~ 1 + a- cos 6


1 -




where E and Ev are the total energy of the emitted 3 particle and the neutrino, is the angle between

Experimental uncertainties of past determinations of the [—v angular correlation coefficient a (The top part shows the measurements that determined that the weak interaction is of predominantly V and A type. The bottom part lists the best limits for S- and T-type interaction to date. They are at the level of 6% at best (68.3% C.L.). This corresponds to a mass of the exchange boson mboson « « 300 GeV when standard coupling is assumed.)

The coefficients Ci,C'i, i e{S,V,T,A,P}, are the coupling constants for the S, V, T, A, and P interaction types. Gf is the Fermi coupling constant and Vud is the ud-matrix element of the CKM matrix.

The presence of S- and T-type interactions is not yet experimentally ruled out with high precision. Most

Nuclide A a Reí. Comment

6He 0.05 [3] V - A structure

19 Ne 0.08 [3] V - A structure

23 Ne 0.04 [3] V - A structure

35 Ar 0.14 [3] V - A structure

6He 32 Ar 0.003 0.0065 [4] [5, 6] Ct/Ca < 6%(C.L. =68.3%) Cs/Cv < 8% (C.L. = 68.3%)

the 3 particle and the neutrino, m is the rest mass of the electron, r = y/l — (aZ)2 with a the fine-structure constant and Z the nuclear charge of the daughter nucleus, v/c the velocity of the 3 particle. The Fierz interference coefficient b has experimentally been shown to be small [8—10] and will be assumed to be zero.

The /—v angular correlation coefficient a depends on the coefficients Ci, C'i. For pure Fermi (F) and pure Gamow—Teller (GT) transitions this dependence can be approximated as:

Intensity, arb. units

aF — 1 —

\Cs i2 +


aGT -


\Cv I2

\CT\2 +





In the SM, i.e., in the absence of S- and T-type interactions, aF = 1 and aQT = —0.333. Any admixture of S to V interaction in such a pure Fermi decay would result in a < 1. The measurement of a therefore yields information about the interactions involved. However, the neutrino cannot be detected directly in such an experiment and the ¡3—v angular correlation has to be inferred from other observables. From properties of the general Hamiltonian of the weak interaction [11] and of the Dirac y matrices it can be shown that V interaction only takes place between a particle and antiparticle with opposite helicities while in case of S interaction only between a particle and antiparticle with the same helicities. Therefore in superallowed 0+—0+ Fermi decay, where ¡3 and neutrino spins have to be coupled to zero, particle and antiparticle will be emitted preferably into the same directions for V interaction and into opposite directions for S interaction. This will lead to a relatively large energy of the recoil ion for V interaction and a relatively small recoil energy for S interaction (Fig. 1).

The aim of the WITCH experiment is to improve the limits on S- and T-type interactions. For this the ¡—v angular correlation will be determined with high precision by measuring the shape of the recoil energy spectrum. The table shows the uncertainties of a achieved in other experiments so far. Consequently, the WITCH experiment has to reach a precision of Aa < 0.01 to achieve its objective.


An experiment to measure the shape of the recoil energy spectrum has two difficulties: 1) the 3 emitter is usually embedded in matter, which causes a distortion of the spectra due to the ion scattering

0 200 400

Recoil energy, eV

Fig. 1. Differential recoil energy spectrum for a = 1 and a = -1.

in the source, leading to energy losses; 2) the recoil ions have very low kinetic energy making a precise energy measurement difficult. In order to avoid the first problem and to be as independent as possible from the properties of the isotopes to be used, the WITCH experiment will use a Penning trap to store radioactive ions. An ideal Penning trap can be defined as the superposition of a homogeneous magnetic field B and an electrostatic quadrupole field U, coaxial to the magnetic field. The combination of these particular fields allows to store a charged particle in a well defined volume. Also, there is an exact solution of the particle's equations of motion [12].

The Penning trap has to capture, cool, center and store the ions. To separate these functions in the WITCH setup two Penning traps are used, a cooler trap and a decay trap (Fig. 2). First, the ions get trapped in the cooler trap, where they are cooled down to room temperature and centered. For this purpose helium gas is injected into the trap as buffer gas. Then the ions are ejected through a differential pumping barrier into the decay trap where they are kept for a couple of half-lives, i.e., of the order of 1 s to 10 s for the cases of interest. The ion cloud in the decay trap constitutes the source for the experiment [13].

To solve the second problem and measure the recoil energy spectrum a retardation spectrometer will be used. The working principle of such a device is similar to the ¡-spectrometers used for the determination of the neutrino rest-mass in Mainz [14] and Troitsk [15]. The spectrometer consists of two magnets of Bmax = 9 T and Bmin = 0.1 T and an electrostatic retardation system. Recoil ions are created in the strong magnetic field region and pass on their way the region with low magnetic field. Provided that the fields change sufficiently slow along the path of the ions, their motion can be considered as adiabatic.





Fig. 2. Geometry of the Penning traps. The cooler trap (left) is separated from the decay trap (right) by a differential pumping barrier. The total length of the two traps is 42.8 cm. EE denotes the End-cap Electrodes, CE — the Correction Electrodes, and RE — the central Ring Electrode of the traps. Also the buffer gas inlet is shown.

According to the principle of adiabatic invariance of the magnetic flux, a fraction 1 — (Bm;n/B

max) ^

« 98.9% of the energy of the ion motion perpendicular to the magnetic field lines will be converted into energy of the ion motion along the magnetic field lines. Then in the homogeneous region of low magnetic field Bmin the total kinetic energy of the recoil ions can be probed by retarding them with a well-defined electrostatic potential. By counting how many ions pass the analysis plane for different retardation voltages, the cumulative recoil energy spectrum can be measured.


An overview of the experimental setup is shown in Figs. 3 and 4. In a first step the ions produced by the ISOLDE facility [16, 17] (CERN) get trapped by REXTRAP [18], a Penning trap which serves to bunch, cool and purify the ion beams for the REX-ISOLDE project. Then they are transmitted through the horizontal beamline and a 90°-bender with spherical electrodes into the vertical beamline of the WITCH setup. There the ions are electrostatically decelerated from 60 keV to 50 eV in several steps to enter into the cooler trap. In the cooler trap the ion cloud is prepared to be ejected through the differential pumping barrier into the second Penning trap, the decay trap. The latter is placed at the entrance of the retardation spectrometer. After 3 decay the recoil ions emitted into the direction of the spectrometer spiral from the trap placed in the strong magnetic field into the weak field. In the homogeneous low-field region the kinetic energy of the ions is probed by the retardation potential. The ions that pass this analysis region are re-accelerated to ~10 keV to get off the magnetic field lines. The re-acceleration also ensures constant detection efficiency for all

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