Lawrence Livermore National Laboratory



Ate Visser (16-FS-016)

Abstract

The observation of a permanent electric dipole moment (EDM) in an atom would signal the long-sought-after presence of time-reversal violation and would have far-reaching consequences for both particle physics and cosmology. The primary challenge for an EDM experiment using radon is to collect large enough samples to perform sensitive searches. One promising way to collect large samples of radon isotopes is to extract them from the aqueous beam dump at the Facility for Rare Isotope Beams (FRIB) at Michigan State University in East Lansing, which will produce unprecedented quantities of radioactive nuclei when it comes online in about five years.

The objective of this study was to assess the feasibility of harvesting radon and noble gas isotopes from the FRIB cooling-water system. To that end, we conducted a detailed study of the FRIB design and an evaluation of the behavior of noble gas and radon isotopes in the FRIB cooling system. We participated in the 4th FRIB Workshop on Isotope Harvesting, organized by Michigan State University at the FRIB construction site to exchange knowledge with on-site engineers and collaborating scientists.

Based on the design specifications, we believe it is feasible to harvest selected radon isotopes (211Rn, 221Rn, and 223Rn) from the FRIB beam-dump cooling system. Harvesting 225Rn is not feasible because of its short five-minute half-life. Radon can be harvested on-line by extracting gases from the water passing through the cleanup loop using a membrane contactor to remove all gases from the water and subsequent cryogenic traps to purify specific noble gases. Off-line harvesting of the longer-lived noble gas isotopes radon-211 (211Rn) and krypton-76 (76Kr) from the gas loop is feasible and will yield useful quantities of these isotopes.

Background and Research Objectives

According to current theory, approximately 15 billion years ago matter and antimatter were created in equal amounts during the Big Bang event. It is not known why all the matter and antimatter did not annihilate each other in the intervening years and leave behind a universe nearly devoid of matter such as stars and planets. In the Standard Model of particle physics, there is no mechanism that leads to a significant excess of matter over antimatter. Therefore, something substantial must be missing in the theory. The observation of a permanent electric dipole moment (EDM) in an atom would signal the long-sought-after presence of a mechanism known as time-reversal violation to explain matter–antimatter asymmetry (Lees 2012). An electric dipole moment (EDM) is a measure of the separation of positive and negative electrical charges in a system of electric charges. The most promising place to observe this mechanism in action is thought to be in nuclei with an octupole-deformed shape (i.e., a nucleus with eight poles), such as the short-lived (25-minute half-life) radioactive isotope radon-223 (223Rn). The observation of a permanent electric dipole moment in such an atom would have far-reaching consequences for both particle physics and cosmology.

There is great interest in searching for EDMs in octupole-deformed nuclei (Gaffney et al. 2013). All of the octupole-deformed nuclei of interest are radioactive and therefore available only in limited quantities. The objective of this study was to assess the feasibility of harvesting radon and noble gas isotopes from the cooling-water system of the FRIB at Michigan State University. The primary challenge for an EDM experiment using radon is to collect large enough samples to perform sensitive searches. One promising way to collect large samples of radon isotopes is to extract them from the aqueous beam dump (Figure 1) at the FRIB, which will produce unprecedented quantities of radioactive nuclei when it comes online in about five years.


Figure 1.
Schematic diagram showing the optimal location (green box) for inline radon harvesting from the cooling water and off-gas system of the beam dump at the Facility for Rare Isotope Beams (FRIB). Flow rates (L/s) in the cooling water and cleanup loops, and nitrogen (N2) and oxygen (O2) gases present at specific steps in the gas loop are also shown. Note that radon harvesting takes place in the low-flow (1.6 L/s) cleanup loop..

Other noble gas isotopes are of interest for collection at FRIB, besides the heavy short-lived radon isotopes (221Rn, 223Rn, 225Rn) for EDM research. For example, 76Kr is the parent to bromine-76 (76Br), which is used in positron emission tomography (PET) but difficult to produce. 211Rn can be used to produce pure astatine-211 (211At) for alpha-particle therapy. The feasibility of harvesting 76Kr and 211Rn was also included in this study.

The feasibility study objectives were met by detailed examination of the FRIB design and model evaluation of the behavior of noble gas and radon isotopes in the FRIB cooling system. We participated in the 4th FRIB Workshop on Isotope Harvesting, organized by Michigan State University at the FRIB construction site to exchange knowledge with on-site engineers and collaborating scientists. Additional experiments to purify naturally occurring radon were conducted using the krypton-purification system at Lawrence Livermore's Nuclear and Chemical Sciences Division, after modifications for radon, and tested with a Durridge RAD7 radon detector.

Scientific Approach and Accomplishments

Five parameters control the rate at which noble gas and radon isotopes can be harvested from the FRIB cooling system.

  1. Production Rates (RP)
  2. Residence Time and Decay (Fd)
  3. Gas-Water Partitioning (Fw)
  4. Cleanup Loop Fraction (Fc)
  5. Extraction, Trapping, and Purification Efficiency (Fe, Ft, Fp)

The net harvesting rate (RH) is the multiplication of these parameters (see Equation 1).

RH = RP × Fd × Fw × Fc × Fe × Ft × Fp Equation 1

The net harvest rates (RH) for krypton and xenon from the water in the cleanup loop are calculated using Equation 1. Harvest rates for radon isotopes are 6.7 × 106 (221Rn), 1.8 × 106 (223Rn) and 1.2 × 105 (225Rn). These correspond to 0.024, 0.023, and 0.0010 (respectively) of the production rates in the beam dump. Decay inside the water cooling system is a major loss term for 225Rn. If radon is continuously harvested online, the isotopes will reach saturation equilibrium (Aeq) on the cryogenic trap when trapping and decay rates are equal (teq). The equilibrium (99 percent) is reached after 166, 154, and 30 minutes, resulting in 1.4 × 1010, 3.6 × 109, and 4.7 × 107 atoms on the trap for 221Rn, 223Rn, and 225Rn, respectively.

Longer-lived isotopes 76Kr and 211Rn reach saturation equilibrium in 5,900 and 5,820 minutes (approximately four days). After 24 hours, 6.6 × 1012 atoms 76Kr (radioactivity = 2 mCi) and 5.6 × 1013 atoms 211Rn (14 mCi) would have accumulated. While it is possible to harvest the longer-lived isotopes from the water loop using the same equipment as that used to harvest the short-lived radon isotopes, larger quantities can be harvested from the FRIB's gas phase.

Off-line harvesting of long-lived noble gas isotopes from the gas phase is the preferred method. Henry’s Law air-gas partitioning controls the preference for noble gas isotopes to accumulate in either the gas phase or the water phase. Krypton isotopes favor the gas phase more strongly than radon isotopes, therefore long-lived krypton isotopes accumulate in the gas phase.

Both 76Kr and 211Rn will accumulate over the course of four days before reaching saturation equilibrium at 4.5 × 1014 atoms 76Kr (160 mCi) and 9.5 × 1014 atoms 211Rn (340 mCi). Harvesting is not time-constrained and isotopes can be allowed to accumulate in the gas phase for as long as the beam is operational. Shorter accumulation times (e.g., 24 hours) still yield useful quantities of these isotopes (68 percent of equilibrium). Radon, krypton, and other noble gas isotopes can be purified in a sequence of adsorption–desorption steps on sequentially smaller cryogenic traps, based on existing technology.

Impact on Mission

The search for electric dipole moments is anticipated to be a major high-profile research effort at the Facility for Rare Isotope Beams. Our study is well-positioned to bring Lawrence Livermore expertise and capabilities together to investigate how to safely and successfully collect the radon needed to realize this major research goal. Our study strengthens the nuclear, chemical, and isotopic science and technology core competency, and supports the Laboratory’s missions in stockpile stewardship and related areas by delivering relevant new technological capabilities.

Conclusion

Based on the design specifications, it is feasible to harvest selected radon isotopes (211Rn, 221Rn, 223Rn) from the FRIB beam dump cooling system. Harvesting 225Rn is not feasible because of its five-minute half-life. Radon can be harvested on-line by extracting gases from the water passing through the cleanup loop using a membrane contactor (to remove all gasses from the water loop) and subsequent cryogenic trapping to purify specific noble gases. Online harvesting from the gas loop is not feasible, due to the dimensions of the gas flow and required cryogenic trap. Off-line harvesting from the gas loop is not feasible for shorter-lived radon isotopes (221Rn, 223Rn), due to the total volume of gas, lower radon concentration, and time constraint imposed on off-line harvesting by the radon half-life. Off-line harvesting of longer-lived noble gas isotopes (211Rn, 76Kr) from the gas loop is feasible and will yield useful quantities of these isotopes.

References

Gaffney, L. P., et al. (2013). "Studies of Pear-Shaped Nuclei Using Accelerated Radioactive Beams." Nature 497: 199—204. doi: 10.1038/nature12073.

Lees, J. P. 2012. "Observation of Time-Reversal Violation in the B0 Meson System." Physical Review Letters 109 (21). doi: 10.1103/PhysRevLett.109.211801.