Summary of the efficiency of a double MOT system

The collection efficiency of a MOT system which loads from a vapor originating from a neutralized ion beam, depends on three factors: the neutralization efficiency, the single pass capture efficiency and the number of times the atoms pass through the trap volume before they are lost. The neutralization of the ion beam can be best done with a neutralizer foil. An efficiency of about 50% can be expected for a temperature of 1200 K. The single pass trapping efficiency directly depends on the loading rate of a MOT system. An analytical calculation which oversimplifies the slowing process showed that the commonly used loading rate overestimates the true loading rate with a factor of 2. Using a 3D Monte Carlo simulation for a Na MOT, with a known capture velocity, we could establish that the loading rate is overestimated with about a factor of 3.

To calculate the capture velocity from the loading rate also the loss rate in a MOT system due to collisions with the vapor needs to be known. Using a classical model we found good agreement between the calculated loss rate and experimental data obtained with a Rb MOT.

Having tested both loss and loading rate, we calculated the capture velocity for a vapor cell MOT system trapping alkaline elements. We found that from all alkaline isotopes Na has the lowest capture velocity. An explanation may be found in the details of the hyperfine splittings in the excited state.

A non-stick coating reduces the sticking time of the particles on the wall and therefore allows for multiple trap passages before the particles get definitely lost. An extensive search in literature on efficient trapping shows that the exact conditions required to achieve a large number of bounces are unclear. We find indications that the non-stick coating might actually be covered with a layer of the alkaline of interest.

The quality and condition of the coating might be critical in the case of Na. A possible diagnostic to determine the state of the coating can be provided by Light Induced Atomic Desorption (LIAD).

To estimate the number of bounces and the number of passages the atoms make through the laser trap volume we performed Monte Carlo simulations of the effusion process. We find good agreement of our calculations with experimental data for different geometries from which the number of bounces could be extracted.

We conclude that to achieve an overall capture efficiency of 1% two ingredients are crucial:

2.10 Summary of the efficiency of a double MOT system 51

• A non-stick coating which reduces the sticking time to a negligible timescale.

• A cell geometry in which the atoms pass on average the trap volume about 100 times before exiting.

We decided to implement the near-resonant transfer scheme, which has shown high transfer efficiencies for other alkalines for the transfer from the capture to the science MOT. For the on resonance push beam solution two ingredients are required to achieve a 50% transfer efficiency:

• A resonant push beam, either continuous or pulsed.

• An optical funnel or a permanent magnetic guide to reach the level of 50%.

C

3

Experimental setup

In this chapter the experimental setup of the double Magneto-Optical Trap (MOT) system for²¹Na, which is coupled to an online production and separation facility, is described. In figure 3.1 an overview of the setup is shown. A high-energy ion beam collides with a gas target and the various particles are separated in the TRIμP dual magnetic separator. The particles are then stopped in a stack of heated foils: the Thermal Ionizer (TI). The ions are extracted from the TI and transported at low energy through the Low Energy Ion Beam (LEBL) towards the collector MOT section. The setup for the²¹Na experiment consists of two MOT systems, separated by 69 cm. The first MOT setup consists of a glass cell and collects the neutralized ions. The trapped atoms are then transferred to a second MOT setup where the decay measurement will take place. To enhance the efficiency of the transfer process, optical access is provided by six-way cross half-way the transfer line.

In section 3.1 we introduce the production of²¹Na and the stopping of the high-energy radioactive ion beam in the TI. The setup for the LEBL is described in section 3.2. In the collector cell setup the ions are neutralized, and after being evaporated, trapped optically in a MOT. We introduce the four laser systems that have been used for the trapping of radioactive²¹Na and stable²³Na in section 3.3. The frequency calibration of spectroscopy signals, used to lock lasers, is discussed in section 3.4.

The glass cell that was used for the collector MOT setup is described in section 3.5.

Section 3.6 contains the description of its successor, the cubic cell setup. In section 3.7 the push beam and the optical funnel which were used to transfer the atoms are discussed. The MOT system, which is situated in the vacuum chamber in which the correlation coefficients will be measured, is treated in section 3.8. In section 3.9 a brief description of the data-acquisition system is given.

3.1 Production, stopping and extraction of

Na

The²¹Na for our experiment is produced in the inverse d(²⁰Ne,²¹Na)n reaction at an energy of 22.3 MeV/u. The reaction products have velocities corresponding to the

53

x 10

1m

TRIμP dual magnetic separator

Thermal Ionizer (TI)

21Na double MOT setup Low Energy Beam Line (LEBL)

Gas target

20Ne beam

Figure 3.1: The top view of the experimental setup for the²¹Na experiment.