Laser isotope separation to study for the neutrino-less double beta decay of 48Ca

The laser isotope separation (LIS) system is being developed for the study of neutrino-less double beta decay of 48Ca. A proof-of-principle experiment has been successfully conducted, and development of a mass production (∼1 mol/year) system is currently underway. For stable and efficient production, we are preparing an atomic beam system capable of producing a well-collimated beam with sufficient flux and a high-power semiconductor laser system with stable frequency over a long period of time.


Introduction
To verify the Majorana nature of neutrinos, double beta decay without neutrino emission (0 decay) experiments have been performed on several nuclei, among which 48 Ca is characterized by the largest Q value (4.27 MeV) and a small natural abundance (0.187%).The former allows for the low background measurement [1], while the latter makes isotope enrichment essential to increase the experimental sensitivity.For example, to be sensitive to the effective neutrino mass (0.03 eV) in the inverse hierarchy region, we require 0.36 − 13 kmol of 48 Ca (depending on the nuclear matrix elements [2]), assuming background-free conditions, a total detection efficiency of 0.8, and a measurement time of 10 years.We have developed a 48 Ca enrichment method using LIS [3].Calcium has a transition (wavelength of 422.792 nm) with an isotope shift of a few hundred MHz.This transition can be used to selectively excite specific isotopes with a commercially available blue-violet diode laser (DL).We applied the deflection method using this transition.By absorbing and emitting multiple photons, the momentum of the laser beam is transferred to the atom, which is then separated from the original atomic beam.We have finished a proof-of-principle (POP) experiment [4] and are constructing a system capable of producing about 0.1 − 1 mol/year of enriched 48 Ca.

R&D for mass production system
Figure 1 shows a schematic drawing of the production system currently under construction.Six atomic beams are generated vertically upward in a vacuum chamber.Selected isotope ( 48 Ca) atoms are deflected by lasers entered through windows on the sides of the chamber.The enriched 48 Ca and depleted calcium are collected separately at the top of the chamber.The goal of this system is to produce 1 mol/year of 48 Ca by operating six atomic beams, and the first step is to achieve stable operation of one of 6 beams.Based on calculations in the reference [4], the required evaporation rate of one atomic beam is 1.6 g/hour and the laser power is about 6 W. The value of evaporation rate can be achieved by heating the crucible that contains the calcium atoms to 870 K, assuming that the area of the beam generator is 5 × 180 mm 2 and all the evaporated atoms form the beam.
For the mass production, the important parameters are both the enrichment and the collection efficiency, which is the ratio of the 48 Ca atoms collected to those in the raw calcium set in the crucible.Optimization of these parameters requires a well-collimated atomic beam (~zero momentum perpendicular to the beam axis direction) and the laser with stable power and oscillation frequency over a long period of time.In addition, long-term and stable production requires an overall automatic control system, including a monitoring system for various parameters.These are under development using the prototype system used in the POP experiment, as described in sec.2.1 and 2.2.

Atomic Beam System
The atomic beam generation system consists of an electrically heated crucible containing natural calcium metal and a tube-type collimator.This simple system ensures long-term stable operation and easy maintenance for mass production.The capacity of the crucible was increased to about 50 times that of the POP experiment.The atomic beam flux can be stabilized by controlling the crucible temperature, and the amount of the flux is monitored by a thickness sensor.For the measurement of the spatial distribution of the atomic beam, we continue to use the TOF system that was used in the POP experiment and are also developing a beam profile monitor that measures the fluorescent light emitted from atoms irradiated by the deflecting laser.By sweeping the laser frequency, the velocity profile in the laser direction can also be monitored using the Doppler effect.On the other hand, if the atomic beam is sufficiently stable, a shift in the wavelength of the laser oscillation can be observed as a shift in the peak position of fluorescence.We are developing the system to stabilize the wavelength of the master laser in the laser system described in next subsection.
The effect of the length of the single-tube collimator on the atomic beam spread was reported in Ref. [4].As the next step in generating a sheet-like atomic beam, a collimator with three tubes in a row was created and set in a crucible.The stability of the intensity and the spatial distribution of the beam generated by this collimator are now being investigated.Based on these experimental results, we are also developing a simulation code that takes into account the interaction of calcium atoms with laser photons, collimators, and other structures.A laser is irradiated perpendicular to the beam, and only 48 Ca is deflected and collected by a collection plate.The depleted atomic beam is also collected by another plate.

Laser System
The laser performances required to produce the amount of enriched 48 Ca for the 0 experiment are: 1) stable (< 2 MHz rms) continuous wave oscillation at a calcium absorption wavelength of 422.792 nm , 2) power scalability from 10 to 1000 W per unit, 3) long lifetime exceeding 30,000 hours, and 4) low cost.We are developing the laser system that uses multiple gallium nitride (GaN) laser diodes (LDs) as the light source to achieve both power scalability and stable oscillation at a single frequency.For the former, the output power per element is increased using techniques such as semiconductor optical amplifiers (SOAs), while the total system output is increased by using multiple slave lasers (SLs) that are injection-locked by a wavelength-stabilized master laser (ML).If the power of the ML, which is an external cavity laser diode (EC-LD), is high enough, multiple SLs (Fabry-Perot laser diodes: FP-LDs) can be locked by the injected signal divided from the ML.In this way, power scaling can be achieved by increasing the number of FP-LDs.
Figure 2 shows the result of an oscillation wavelength stabilization experiment in the injection locking system [4] consisting of the ML and single SL.The oscillation wavelength of the ML was stabilized by controlling the angle of the grating of the EC-LD with a high-precision laser wavelength meter, while that of the SL was locked by controlling the current of FP-LD with the Pound-Drever-Hall (PDH) method.The relative frequency fluctuation of the SL stabilized by the PDH signal was measured to be 0.6 MHz rms in 3 hours, much smaller than the natural broadening of the Ca atomic beam.As a next step, the number of FP-LDs will be increased to demonstrate the output scalability of the injection locked system.We are designing a device that integrates multiple SLs, where each SL consists of an FP-LD, an optical system including a beam splitter that distributes the incident light from the ML, and an LD holder with a temperature control mechanism.

Summary and Outlook
The mass production system for enriched 48 Ca using the laser deflection method is being developed to study neutrino-less double beta decay.In the atomic beam system, beam collimation with multiple tubes is used to produce a sheet-like atomic beam of 1.6 g/hour.The injection-locked laser system uses relatively inexpensive, commercially available LDs, aiming for an output power of > 6 W per unit.By combining these with the collection system and the automatic control system, the production system will first be operated at a rate of 0.1 to 0.2 mol/year, after which the atomic beam-laser system will be increased to six units to achieve a production of 1 mol/year.This value will be then increased by increasing the atomic beam and laser intensity, and by preparing multiple units, the goal is to eventually achieve a production rate on the order of kmol/year.

Figure 1 .
Figure 1.(Left) Schematic diagram of the production system currently under construction.(Right) Conceptual view from the side.A crucible installed in the lower part of the apparatus is heated to generate beams of calcium atoms through tube-type collimators.A laser is irradiated perpendicular to the beam, and only48 Ca is deflected and collected by a collection plate.The depleted atomic beam is also collected by another plate.

Figure 2 .
Figure 2. Time variation of the oscillation wavelength of the injection-locked laser system measured by a wavelength meter; the EC-LD was stabilized by the wavelength meter and the FP-LD by the PDH method.The frequency fluctuation was about 0.6 MHz rms in a 3-hour measurement.