Barium Ion Tagging : Ion Acquisition in LXe & Laser Fluorescence Identification As has been outlined in the plenary talk, the tonne-scale EXO experiment can reach the 10 meV mass scale exploiting the dramatic background reduction provided by coincidence tagging of the barium daughter of xenon double beta decay. The ongoing R&D program will be summarized here. P.C. Rowson SLAC
X (Y++)* + e– + e– One possibility would be the Observation of a from an excited daughter ion, but the rates compared to ground state decays are generally very small (best chance might be 150Nd, but Eis only 30 keV.) Y++ + A more promising approach : Barium detection from 136Xe decay 136Xe 136Ba++ + e– + e– Identify event-by-event Described in 1991 by M. Moe (PRC, 44, R931,(1991)). The method exploits the well-studied spectroscopy of Ba and the demonstrated sensitivity to a single Ba+ ion in an ion trap. Background reduction by coincidence measurement It was recognized early on that coincident detection of the two decay electrons and the daughter decay species can dramatically reduce bkgrd.
Barium Tagging R&D • Our decision to proceed with a LXe TPC (as opposed to gXe) • led us to investigate ion retrieval, or “ion to laser”, schemes • for barium tagging. • So far, this work has proceeded in parallel • mainly at Stanford and SLAC : • Laser ion trap program - trap design & operation • Ion capture program - electrostatic probe designs • Interface program* - ion-to-trap transfer • *(recently begun) • In addition, at CSU, W. Fairbank, is investigating : • “in situ” tagging - laser tagging in LXe
Comment on Barium backgrounds Barium atoms hypothetically present in the xenon would not normally constitute a background, as we only collect barium ions. Barium ions from 2 decay are produced in the xenon at a rate not yet determined, but limited to ~300,000/tonne-year, or roughly 1 per 100 seconds per tonne. These are continually swept out of the liquid by the TPC E-field in < 30 seconds for our nominal ~3 kV/cm field strength. (The ion mobility is known - more on this later). ... some preliminary studies … Correlated sources of barium ions have been investigated and appear to be negligible. Rates are low and in addition, event topologies should be distinctive. Detailed MC simulation has not yet been deemed high priority but will be done. Xe136(p,n)Cs136 : Cs136 production by cosmics (Cs→Ba via decay) Xe136(,)Cs136 : Cs136 production by solar neutrinos Xe136(n,γ)Xe137 : Xe137production by cosmics (Xe→Cs→Ba via )
Liquid Xenon TPC conceptual design Compact and scalable (3 m3 for 10 tons). The basic concept, shown here for a LXe option, is : • Use ionization and scintillation light in the TPC to determine • the event location, and to do precise calorimetry. • Extract the Barium ion from the event location (electrostatic • probe eg.) • Deliver the Barium to a laser system for Ba136 identification.
175 nm scintillation e- Ion capture in LXe TPC Basic electrostatic capture procedure Probe motion (3 d.o.f.) triggered by event E threshold. The probe moves above the TPC, and then vertically down to the event location. The Ba+ is collected electrostatically (doesn’t move far from the event location), and the probe is withdrawn. electrostatic probe LXe level cathode TPC charge & UV detection • Issues to be addressed (R&D progress where indicated) : • Ba+ lifetimes in LXe (expected to be long - data exists) • Ba ion drift velocities (should be a few mm/sec - confirmed) • Ba capture and release – various probe designs Ba transport to the laser spectroscopy station
Laser fluorescence barium identification A well-studied technique pioneered by atomic physicists in the 1980’s for the detection of single atoms and ions, in particular, alkali and alkaline-earth metals. Ba++ lines in the UV – convert ion to Ba+ or Ba. “Intermodulation” “Shelving” into metastable D state allows for modulation of 650nm light to induce modulated 493nm emission out of synch. with excitation (493nm) light – improves S/N
Laser Spectroscopy Lab at Stanford red laser reference cavities blue laser Stable and reliable laser system RF applied Ba oven Ion trap : hyperbolic Paul type laser, Ba ionizer and detection line-of-sight through these gaps
Glare from electrodes Single Ba+ signal The trap is loaded with multiple ions: We observe the signal intensity as ions are dropped one by one…
The effects of buffer gas on trap performance The operating environment of the EXO ion trap will likely include some level of background xenon gas, and the effects of this “buffer gas” have been studied. It has been found that the addition of helium can improve trapping times (which are essentially indefinite for UHV conditions for modest xenon pressures. Differential pumping can/will be used to maintain a low ion trap buffer gas pressure.
Ba+ cloud image in liquid xenon Liquid surface 8 mm Grid Concept for Ba+ tagging in the Liquid in a LXe Double Beta Decay Experiment : “laser-to-ion” schemes. ßß Decay then Ba++ Ba+ Laser Fluorescence CCD/APD Filters Slit Focus • At CSU, fluorescence • data in LXe has taken, and • studies are continuing. The • issues here are : • Line broadening/loss of • specificity. • S/N improvement for in situ • ion detection. CCD camera image of Ba+ fluorescence in LXe
Pa produced in a cyclotron 230Th + p 230Pa + 3n 230Pa (17.4d) 8.4%b 230U (20.8d) 5.99MeV 226Th (30.5min) 6.45MeV 222Ra (38s) 3-steps of decay Barium ion extraction R&D at SLAC Ion capture test simulates Ba ions by using a 230U source to recoil 222Ra into the Xenon – Ba and Ra are chemically similar (ionization potentials 5.2 eV and 5.3 eV respectively). 1st Prototype electrostatic probe– W tipped. Variations have been tried (diamond coated), but ions not released by reversed HV in these cases (required E field too high)
outer vac. vessel Probe test cell Xenon cell Probe lowered for ion collection (1) Electrode (source) PMT 3-position pneumatic actuator probe up position for release (3). detector flange counting (2) station Xenon cell
Ion extraction from Xe and LXe 230U source α spectrum as delivered by LLNL (measured in vacuum) αspectrum from whatever is grabbed by the tip (in Xe atmosphere) An additional signature from the observed Th and Ra lifetimes.
Ion mobility studies in LXe We use the probe test cell to measure ion drift speed forward bias LXe level “Paddle” probe U230 source electrode reverse bias Modulate the electrode voltage, and measure ion collection rate. Data taken for various separation distances and voltage differences. Observed mobility of 0.24±0.02 cm2/kVs for Thorium ions compares with result for Thallium ions 0.133 cm2/kVs. (A.J. Walters et al. J. Phys. D: Appl. Phys.) and with Fairbank etal. for EXO (Ba,Sr,Ca,Mg). Our work submitted to Phys. Rev. B.
Argon Expected gas cooling from calculated J-T coefficient and our data with cryoprobe. Ion Capture “Cryo Probe” prototype In order to release a captured ion, the electrostatic probe can be cooled such that Xe ice coats the tip. The captured ion can then be released by thawing. Joule-Thompson cooling is used for cooling (argon gas). An additional benefit : the Ba+ charge state may be stable in solid and liquid xenon. Probe tip detail gas return (outer tube) incoming gas (inner tube) small aperture at tube end Remarkably, surgical cryoprobes seem to be ideally suited to our application. We have adapted 2.4 mm diameter probes for use in our probe test cell.
2.4mm Vacuum jacket J-T nozzle TC X-ray image of new cryoprobe test version of “thaw” heater Testing the ion extraction probe U230 sources were installed, xenon was liquefied in the cell, ion capture and release from Xe ice has been demonstrated. First cryo-probe was not equipped for acceptably “graceful” Xe ice release. New version is under test. Refinement of ion release procedure (rapid ice sublimation is best). Issue for cold probe method - Xe gas release
5.2eV 10eV ~5.9eV Ba Pt (111) work function ionization E Surface ionization or “hot probe” R&D It is well known that heated metal surfaces can release captured metal atoms in both the neutral or ionized state : “impact ionization” Saha-Langmuir effect : Ion emission from heated high-work function surfaces (shown here for alkaline earth metals) known from ion beam experiments May be possible to release Ba+ ions by heating Pt probe. This procedure would be simpler than the cold probe. Method requires the Pt surface is heated to a high enough temperature to efficiently liberate barium, but not so high that neutral atoms become a significant fraction w.r.t. ions.
Test apparatus for thermal ion release experiments • Th228 (1.9 yr) source produces • Ra224 (3.6 d) daughters • Source can be forward or backward • biased (±500 V typ.) • Pt foil (@ ground) receives • ions recoiled from source. • Foil can be moved in front of detector, • and down to the stopper plate. • Foil heated >1000K, see if Ra released • as neutral or charged. • (if the observed post-heating signal is modulated by the HV on the plates, ions were released) heater PS source collimator plate Movable Pt Foil source (Th228) HV Alpha counter Vessel is filled with 1 atm Xe. This limits the diffusion of the ions. The α’s range out in ~5 cm stopper plate HV
At top, the Th228 source Below, the Si SB α detector Pt foil (power leads visible as is the mounted TC) Test apparatus : Source collimator not installed for this photo E field calculation for collimator. Ra224 deposited near foil center
Po-212 Bi-212 Rn-220 Ra-224 Po-216 Red histo: alpha spectrum from foil prepared with reversed biased source → Ra ions do not reach foil. Black histo : … and when source at + potential → foil plated. Experiments are underway in out lab to test the performance of a Pt foil. If promising, we will proceed to design a hot probe, and experiment with different metal tips (iridium is a possibility - higher m.p. than platinum), and perhaps high-work-function dielectrics.
Recently, a third probe option is under study at Stanford - High field emission from “STM” tip, or “sharp probe” R&D Published data suggests that barium will desorb from tungsten needle tips as a Ba+ ion at electric fields of ~150 MV/cm. These high fields can be reached with very sharp STM needle tips (radius of curvature of ~10 nm) at moderate (10 kV) voltages . SEM image of W needle Electric field calculations for ion capture are underway. One of the issues here will be the robustness of these delicate sharp tips
Issues for Trigger rates • event energy & space location from TPC • “ion fetch” triggered by energy threshold & ~veto • TPC field switched off (prior ion drift very small). • move probe tip to (just above) ion location. • capture ion electrostatically with ~1 cm radius. • withdraw probe - TPC field back on - detector live • deliver ion to laser for identification. Acceptable deadtime/Δt for steps 2-6 sets maximum “ion fetch” rate. Our measurements of the mobility of ions (Th and Ba) in LXe indicate a drift speed of ~2 mm/s in a 1 kV/cm E field. For a 1 mm radius probe tip, this translates into a 0.8 s collection time from 5 mm, 3.8 s from 10 mm. The deadtime will be dominated by probe motion and/or high voltage ramping, if necessary - < 1 minute a reasonable target. Backgrounds/trigger threshold sets “ion fetch” trigger rate. While it is difficult to extrapolate from our prototype simulations to a large multi-tonne detector, we can guess by scaling our bkgrd. simulations by a factor of 10 tonnes/200 kg = 50. For a low energy trigger threshold of 2.250 MeV (for an E resolution of 1%, this corresponds to 10σ), trigger rate would be < 1/hour. This is a plausible “ion fetch” rate. (2 events not as important for the large detector - these and other low energy phenomena can be acquired using a scaled trigger).
The probe-to-ion trap interface • We have decided to focus our initial R&D efforts on • an interface between an electrostatic probe and a linear • ion trap, including cryo- and differential pumping. • We have made progress studying electrostatic probes. • A number of issues remain … • The ion-release procedure for the designs considered to • date will have different challenges (assuming the basic • concepts are fully demonstrated in R&D). • The cold probe will deliver a larger Xe load – • Is effective pumping possible ? • The hot probe may release Ba if it is present in the probe • surface material – • Sensitive tests needed during R&D. • …and a bit further down the road … • Significant engineering problems will need our attention – • R&D for probe “robot” and interface to the TPC.
release ions to trap, detect and measure efficiency conventional Ba+ loading detect and count trapped ions capture ions on probe tip 6 mm Linear Paul ion trap R&D TMP/cryo pump probe unloading ion trap/laser tag Linear trap confinement : radially by RF quads, axially by DC fields 600 mm linear trap RF quadrupoles segmented (15 sections) to grade DC axial field. linear trap vacuum chamber (excluding probe interface section) There is considerable experience among nuclear/atomic physicists with ion transport in linear traps. Parts are on order for a linear ion trap to be built at Stanford. R&D continues on trap/probe interface at SLAC/Stanford.
Progress to date • We have developed the atomic physics and spectroscopy techniques to achieve good quality tagging in presence of some Xe gas. • Gained experience with grabbing on Xe-ice and on metal tips • Continuing R&D • Building a linear trap that should be very close to final device • and can be used to test loading efficiency. • Ion release needs more R&D work, field emission from STM-tip, “impact” ionization and “cryoprobe” • all under development in parallel. • Highest present priority/risk Ba tagging R&D must continue in parallel with the construction of the 200 kg experiment in order to move EXO towards the 10 meV regime.