cody h. a new hydrogenicatom, e h and mipbf a new reactor
TRANSCRIPT
A New Hydrogenic Atom, e+H‐
and MIPBF‐a new reactor based e+ source/trap
Jagiellonian Symposium on Fundamental and Applied Subatomic PhysicsKrakow, PolandJune, 2015
Cody H. Storry
The effort
Ivan Guevara (PhD student) Matthew Weel (Post Doc.)Matthew George (Post Doc.)Eric Hessels (Faculty)Richard Thai (Student)Olga Andriyevska (Undergrad)Me (Faculty/Student)
Funding
A “new” Atom, e+ H‐ (or H‐+, name ?)
e+ bound in Rydberg state far from H‐ ion.
e+high n,
Rydberg state
Ground state H‐ ion with two electrons in n=1 S state
Rydberg atomic states are Hydrogeniclifetimes ~10Sec in n~10, high l states
Technique:B= 0.13Tesla
800eV H‐ ion beam
e+ in a Penning Trap, at ‐700V Potential
e+H‐ goes forward (H‐momentum) with 100 eV energy
2 meter
e+H‐ hits surface, e+ annihilates
NaI + PMT
Detect back to back annihilation gammas
Experiments are done at York University (Toronto, Canada)• e+ system at York
– Nearly identical to the e+ system we assembled and operate with ATRAP antihydrogen
– Techniques developed at York support ATRAP – Training for future ATRAP students (Dan wants to come home)
… but also … independent experiments at York (positronic atom research: Ps, e+H‐, …)
e+ system e+ source and moderator
e+ accumulator
e+ annihilation gamma detector:‐energy,‐multiplicity, ‐10cm Pb shielded
York’s Positron programsYork becoming a leader in e+ systems. [PRL 100, 113001 (2008), NJP 14 (2012) 045006, submitted to PLB)]
• Record moderation of High energy e+ (>4% of e+ entering Ne form eV beam)• Long moderator decay times (many days)• Accumulation: 44000e+/sec per mCi of 22Na• ~100% transfer efficiency (10 m distance to cryogenic trap for Hbar)• Highest number ever trapped (4 x 109 trapped, cooled in 1K trap) • Run hands free for weeks of continuous operation (moderator regrowth,
loading, pulsing, synch with variable AD pulse time… automated)
• 3 Positron systems from York University– 1 e+ for Hbar at CERN with ATRAP– 2 Positronic atoms at York (this talk)– 3 Nuclear reactor based e+ source at McMaster U. near Toronto.
(York now leading e+ source, guide, switch, beam‐lines as well as accumulator and experiments)
A bit more about the Reactor system
Intense e+ source by pair production
Guide e+ beam with B fields
Switched between 4 e+ beam-lines (experiments)
One for slow trapped e+ experiments
P. Mascher, McMasterUniversity, Project Leader
S.E. Day, A.P. Knights, P. Kruse, J.S. Preston, S. McMaster, J. Wojcik
P.J. Simpson, University of Western Ontario,
M. Weel, O. Andriyevska, R. Thai, A Leung, C.H. Storry
MIPBF(McMaster Intense Positron Beam Facility)at the McMaster Nuclear Reactor
McMaster is in Hamilton, not far from Toronto (1 hour).
A University Nuclear Reactor:
-essentially continuous operation since 1959.
-16 hours x 5 days at 3MW (5MW)
-used for a wide variety of educational, research and commercial purposes, including medical isotope production.124Xe (n,γ)→ 125Xe→125I ($ saved the reactor)
Today
Under construction 1957
In the core, gas chambers are inserted daily for medical isotope production and radial beam ports with direct access to the core (20cm diam ports)
Beam ports:1 Radiography (industrial)2 Radiography (industrial)3 Radiography (research)4 Prompt‐gamma neutron
activation analysis5 low energy e+ Beam6 Neutron Scatteringe+ beam
MIPBF includes the e+ source, beams and 3 approved experiments (switched)
• SURFACE ANALYSIS (Positron annihilation‐induced Auger electron spectrometer)
• DEFECT PROBE (Coincidence Doppler‐broadening Spectroscopy)
• BUFFER GAS ACCUMULATOR (large numbers of e+ for atomic physics – eg. Ps , e+H‐ research developed at York)
2 operational and 2 future reactor e+ beams
POSH NEPOMUC NCSU MIPBF
Reactor Type MTR-pool Compact high flux PULSTAR MTR-typeReactor Power 2 MW 20 MW 1 MW 3 MW
Reflector H2O D2O H2O H2O
Φth (/cm2/s) 7x1012 2 x 1014 (@ tip of beam tube) 2.5 x 1012 6 to 8 x 1012
Φth:φf 1 to1.6 >104 NA 0.7 to 1.3
Source Generation II II I Design
Beam-tube geometry Radial straight Radial angled Radial angled Radial straight
Beam-tube OD 6” 6” (outer tube) > 8” 8”
Target Material W Cd/Pt W Pt
Target Geometry Grid Plate & Sleeve Grid Plate & Sleeve
Target OD 4” (10 cm) 2.6” (6.5cm) 9” (23cm) 7” (17.8cm)
Target foil thickness 0.05 mm 0.15 mm 0.25 mm 0.15 mm
Cd Converter No Yes Yes No
Cd thickness - 3 mm 0.2 mm -
Primary slow e+/s - 9 x108 ~7x108 Target ~ 5x108
Remoderated e+/s 4x108
(∆E~100 eV at target)5 x 107 - -
Step 1, change beam tube (first time in 23 years)
Reactor fully operational again after a couple of days
Edge of reactor core
Platinum foils: gammas in foil
produce e+/e- pairs. ~15cm diameter.
Solenoid (>0.01 Tesla) to guide low energy e+ out of poolPassively cooled by air (requirements at the reactor)
Step 2: Mount e+ Production Foils Near Reactor Core
Water (pool)
Air
vacuum
Thermal e+ accelerated by applied potentials on foils and a drift tube
e+ trajectories
e+ trapping: use front production foil, formed to match the Bfield curvature. Minimizes transverse e+ energy, B mirror.
If e+ energy width is unimportant (material science) other foils can increase e+ # and to focus the beam
McMaster Intense Positron Beam Facility (MIPBF)
Uranium reactor: gamma source
e+/e‐Pair‐production in Pt foil.
e+ transport in H2O pool
neutron absorber, biological shield
e+ 4‐way beam switch
e+ beam lines (0.02Tesla, KV)
Reactor beam monitor and e‐ gun Buffer gas
accumulator
Accumulator/experiment electronics
Cryopump Compressors and magner power supplies in mechanical room
Defect probe
Buffer Gas e+ Accumulator Ready
e+ buffer gas accumulator-”free” e+, no 22Na required
Accumulator is operational and tested with e-. Since reactor e+ system progressing slow, we took over the development
Load e+ and prepare for e+H‐ production(inside the buffer gas accumulator, buffer gas off)
‐107 e+ accumulated
‐e+ beam and N2 buffer gas switched off (reduce e+ annihilations)
‐e+ Penning trap deepened to ‐700 V
e+ loading and counting
H‐ ion beam
H‐ Ion Beam SourceColutron Ion Source
(used last for my early graduate work)
H2 (~1 torr) hit by 100eV e‐ , produce H‐
Accelerated from 1mm anode hole at ‐800V potential
Focused with electrostatic lens
H‐ separated (from eg. e‐) with E x B momentum filter
50 nA (20 typically) ions directed through 3mm hole, 5 meters from source. (better than manufacture spec.)
~30 days continuous operation (filament)
• long lived neutral e+H‐
atoms continue 2 meters (14s) into a lead shielded detector system
• Atoms hits a metal plate• e+ annihilates• Back‐to‐back gammas
detected and energy analyzed.
‐H‐ ions directed through the trapped e+ and slowed to 100eV by the trap potential
Penning Trap Potentials are adjusted for production/detection
e+H‐ production (some numbers)
107 e+ in Penning trap (‐700V)
800eV H‐Slowed to 100eV at e+
Neutral e+H‐ goes straight(~105 m/s)
e+H‐ hits surface ~14S after production,e+ annihilates
NaI + PMT
Detect back to back annihilation gammasFor a control experiment Ions are
not passed through the e+
Only coincidence counts in the right energy are counted
The expected rate is low so the lowest possible background is key to detecting these atoms
Coincidence detector electronics traditionally: amplifiers, discriminators (energy analysis), delay lines, AND gates (coincidence detector), counters…We implemented a new technique. Store raw PMT signals. Analyzed later.Continuously save PMT signal to computer using fast ADC cards ( 10MHz)Analyze data (and fit signals) later in a variety of ways (eg try different energy cuts)
Powerful:‐Maximize small signal rate ‐Check each gamma in detail‐Optimize the “coincidence” parameters
Actually only store data if the signal is “non‐zero” (reduce stored data)Fitting the data allows us to get the most from a little bit
Tighten coincidence parameters (eg, 100nS sample time not the limit in coincidence time window
No gamma is lost even with overlap
Test source coincidence events vs time window
Can charged e+ reach detector without e+H‐?e+ excited out of trap, go directly to detector (charged particle).
‐e+ would need to gain 1500 eV of energy‐diverging Magnetic field between production and detector (most e+ directed
away from e+ detector)
e+ trapped in ‐700V well at 0.13Tesla magnetic field
800 V barrier
Bfield is low at detector. e+ follow Bfield linesmost are away from detector
NO
Can Ps atoms transport e+ the detector?
Possible Ps states:
2 hyperfine ground states:Ground states have 125pS and 140nS lifetimes for the 1S and 3S states.
Decay and annihilate much too quickly to survive to the detector.
Can e+ get to the detector as metastable or Rydberg Ps?
Metastable state (n=2 3S state) and Rydberg states can have “long” lifetimes ( >1 s )?Momentum of Ps randomly directed (both e‐ and e+ motion random)(spherical S state e‐ in H‐, thermalized e+ motions in the trap)
For this to be our signal would require physically unrealistic scenario. 100% of the e+ converted to Ps.10% of these in metastable or Rydberg statesthese make it out of trapped e+ without charge exchanging to
short lived states. Ps kinetic energy > 30eV13.6eV + 0.75eV binding energy of H‐15eV additional energy required from collision
energy/momentum conservationNO
If e+ are not getting there directly it must be e+H‐ production
2 possibilitiesRadiative combination
Three‐bodycombination
TBR not it (e+ are “hot”, e+ density is low)Must be Radiative combination, rates are consistent with expected rate by this channel
Combine these with expected transit time (100eV, 14s) of e+H‐ to predict the expected signal.
Expected signal rate consistent with our experiment.
Radiative combination cross‐section
Average lifetime of e+H‐ state.
Estimating the signal rate
Predicted rate assumes atoms are directed into annihilation plateSmall amount of transverse energy (few eV) would spread beam to bigger radii than detector. (factor of 5 loss seems possible)
Summary and Outlook for e+H‐
‐Strong indication of a new atom, e+H‐ produced for first time‐Detected rate is consistent with Radiative combination
‐Improve repeatability‐Implementing improved e+ detector (increased collection solid angle)‐Setting up to increase production: (stimulate with CO2 laser: n=11 state)‐Check n state with stark ionization field measurement.