The FrPNC Experiment, weak interaction studies in Francium at TRIUMF

Francium is an excellent system to study the nuclear weak force due to its large nucleus and relatively simple atomic structure. The FrPNC experiment has a facility to produce cold trapped atomic francium samples for parity non-conservation studies. We are preparing to measure both the nuclear spin independent and dependent parts of the weak interaction in francium. The first one gives information about weak neutral currents at low energies, while the second one is sensitive to weak interactions between nucleons. We present the current status of the experiment. 1. Atomic Parity Non Conservation Nuclear forces have been studied traditionally by bombarding the nucleus with a high energy projectile and analyzing the kinematics of the dispersed particles. A different possibility for the study of the weak force is by Parity Non Conservation (PNC) measurements in atoms. This technique is actually not that different from the previous one, since there is still a collision with the nucleus, though weak and continuous, but now the projectile is one of the electrons in the atom. The momentum of the electron in the atom is much smaller to what can be obtained in particle accelerators giving smaller weak interaction strength, but that is compensated in the case of atomic PNC by repeated collisions by the same electron that provides long integration times such that the signal can be extracted with high precision measurements. Atomic PNC measurements are useful for studying the nuclear weak force at low energies (around 1 MeV). XXXV Symposium on Nuclear Physics IOP Publishing Journal of Physics: Conference Series 387 (2012) 012004 doi:10.1088/1742-6596/387/1/012004 Published under licence by IOP Publishing Ltd 1 Atomic PNC measurements look for subtle deviations in the behavior of the electrons in the atom due to interactions with the nucleus. The interactions coming from the nuclear weak force can be separated from other interactions due to the fact that they violate parity. Since time reversal symmetry is preserved at the measured level, the experiments look for the existence of a parity forbidden transition that becomes allowed due to the weak force. The signal grows with a bigger nucleus since the electron has more nucleons to interact with. Moreover, a nucleus with higher Z has a higher electronic density at the nucleus and the electrons have higher velocity. The three effects combine to give a scaling of the (nuclear spin independent) signal faster than Z 3 [1] making heavy atoms the preferred ones. The extraction of weak interaction constants from the measurements rely on atomic structure calculations [2]. Francium is ideal for atomic PNC studies, since it has a large nucleus and a relatively simple atomic structure [3,4]. Furthermore, it features in enhanced signals for testing other fundamental symmetries, such as time reversal symmetry in electric dipole moment (EDM) measurements of the atom [5]. 2. The FrPNC experiment The FrPNC experiment is devoted to the study of weak interactions using francium atoms. We plan to send Fr ions produced in the ISAC facility at TRIUMF to a Y neutralizer foil. After sufficient accumulation of ions we rotate the foil and heat it to release neutral Fr atoms to a high efficiency laser cooling trap [6]. After cooling the atoms, a laser pulse pushes the sample down to a science vacuum chamber where we recapture the atoms in a second trap [7]. We expect to trap an isotopically pure sample of more than 10 6 atoms confined to a small volume (less than 0.1 mm 3 ) and at a very low temperature (around 100 K). The ISAC facility can produce a selection of isotopes in the range of A=203-229. The availability of different isotopes is a particularly interesting feature of the experiment compared to other atomic PNC experiments giving us access to both the neutron deficient and neutron rich francium nuclei. PNC measurements in different isotopes are sensitive to new physics and will help determine neutron skins that will reduce PNC errors induced by them in traditional measurements [4,8]. The long interaction times available with laser cooled atoms allow for coherent measurements that give higher precision, while the absence of atomic motion and the small confinement volume suppress velocity and bias field uniformity related systematic errors. A 500 MeV proton beam from TRIUMF collides with a uranium carbide target to produce Francium. The ions are extracted with 60 kV and the selected isotope is separated at ISAC. The demonstrated yields for the different isotopes produced reaches values up to 10 8 s -1 [9]. The 1% trapping efficiency [6] combined with the production yield gives 10 6 atoms, more than enough for PNC measurements [10]. We have constructed a radio frequency and electromagnetic interference shielded modular room to house the experiment (Fig. 1). The room has an attenuation of 100 dB at electromagnetic frequencies from 14 kHz to 10 GHz that are present in an accelerator environment and also strongly reduces acoustic noise. The temperature and humidity control is currently being installed and should be stable to 0.5 ̊C and 5% relative humidity (at 45%) year round to maintain the lasers in a stable operating environment. The weak interaction in atoms has a nuclear spin dependent and an independent part [11]. The nuclear spin independent part is the dominant one since it adds coherently over all nucleons. The magnitude of this interaction has been exquisitely measured in Cesium by quantifying the transition amplitude between the 6S1/2 to 7S1/2 levels [12]. The measurement together with atomic structure calculations give the value of the weak charge (QW). The weak charge is related to the Weinberg’s angle and atomic PNC gives a value of sin 2 W=0.2382(11) [2] at low energy (below 1 MeV). Francium is heavier than Cesium and the PNC effect is expected to be 18 times larger. The Cesium experiment uses an atomic beam that has the advantage of having a large atomic flux. The number of XXXV Symposium on Nuclear Physics IOP Publishing Journal of Physics: Conference Series 387 (2012) 012004 doi:10.1088/1742-6596/387/1/012004