David B. Cassidy. Department of Physics and Astronomy, University of California, Riverside, USA. Varenna, July 09

Transcription

1 Experimental production of many- positron systems: L2, techniques David B. Cassidy Department of Physics and Astronomy, University of California, Riverside, USA Varenna, July 09

2 Recap: Target chamber Trap Accumulator Buncher Source We saw yesterday how to make intense positron bursts Large positron pulses = 10 to 100 million positrons in ~ 1 ns (~ ma instantaneous current) Detectors typically see around 1 million events, saturation is a problem How do we use these positrons?

3 Large pulses Most detectors useless due to saturation Slow detectors (such as HpGe, NaI) are no use Fast scintillators are often inefficient (e.g., plastic) and very few are fast enough anyway. (ZnO may be useful, but it s not available yet). New detectors had to be found

4 PMT s are easy to saturate

5 Ion/light after pulses After initial photoelectron pulse hits dynodes light and/or ions may be generated that travel back to the cathode and liberate electrons, which causes afterpulsing. This is a bit annoying but not a real problem. MCP might eliminate this

6 Still need a fast gamma-photon converter. Cherenkov radiation is effectively instantaneous. Number of photons emitted per unit length between PbF 2 UV and cathode 50% cutoff dn dx = 1 917cm β n ( λ) Energy loss by ionization de dx 2 1 β ( E + mc ) E 1/ 2 = [0.97 MeV/cm] ln 1/ β I mc Number of Cherenkov photons N = dn dx dx de de N ~ 5. With 50% p.e yield and 25% quantum efficiency you get <1 photoelectron per gammma ray!

7 Fast detectors for single shot lifetime spectra PbWO 4 (Lead tungstate) Scintillator Decay time ~ 15 ns Density 8.3 g/cm 3 Light output ~ 1% NaI PbF 2 (Lead fluoride) Cherenkov radiator Instantaneous light out Density 7.9 g/cm 3 Very low light output < % NaI for 511 kev photons

8 PbF 2 is less sensitive to lower energy gamma rays, and will therefore underestimate Ps fractions (by ~ 1/3). However, it can still be used to detect Ps 10 0 Ps formation on Al(111) crystal PMT output (volts) ion after pulse 375 K 398 K 470 K 500 K Another possibility is to look at 2 and 3 gamma events separately. We haven t tried this yet pre-pulse due to upstream positrons time (ns)

9 How do you know the time width of a large bunched pulse? PbF2 Cherenkov radiator coupled to a Hamamatsu R3809U-50 single channel plate photomultiplier. The single photoelectron response was measured using a small 22 Na test source and is an average of 4096 events Amplitude (arb. un nits) FWHM ~ 260 ps Single photoelectron response Pulse with 20 million positrons FWHM ~ 1 ns time (ns) Is the 1 ns width the real width of the positron bunch or the width of the detector response with many gamma rays? A short pulse laser will answer the question.

10 Lead fluoride and lead tungstate detectors PMT outpu (arb. untis) Primary light feedback pulse (~ 5%) secondary light feedback pulses (~ 0.5%) Ion afterpulses PbF 2 on H PMT PbWO 4 on XP 2020 PMT Al(111) target 10-3 NIM A (2007) time (ns)

11 Single Shot positron annihilation lifetime spectroscopy PALS = one at a time, start-stop counting Good time resolution (150 ps, maybe even less) Cannot be used for intense pulses (the ultimate pile-up) We need to measure Ps annihilation continuously, which is what we call singleshot PALS.

12 Firing sequence for single-shot lifetime acquisition accumulator HV buncher accelerator Pulsed magnet coils scintillator/ Cherenkov crystal PMT target fast oscilloscope stored plasma DC magnet coils 20 ns 20 ms START ka pulse scope trigger 50Ω splitter APL 88, (2006)

13 Low and high gain waveforms are added together from two channels of a fast oscilloscope (6 GHz bandwidth, sampling rate = 20 Gsa/s) Waveforms spliced, inverted, background subtracted and rebinned PMT outpu (Volts) volts high gain waveform (5 mv/div) low gain waveform (100 mv/div) time (ns) time (ns)

14 Conventional and single shot PALS 1 ~ 40% Positronium(TEOS 1 kv) detector output normalised E-3 1E-4 < 2%Positronium(Phosphor screen, 5 kv) From D. Gidley time (ns)

15 How can we make Ps in a short burst? It s actually quite easy to make Ps atoms: positrons directed onto (e.g.,) a porous silica film will make Ps in the bulk which may then diffuse to the voids. They don t care if it s many at once or a short pulse In some samples Ps atoms may also become trapped in a surface state where Ps 2 molecule formation can occur. e + SEQ Ps a-sio 2 Ps 2

16 We can also make Ps on an metal surface: Incident Positron pulse ~ 2 kev Positron trapped in a surface state τ 0.5 ns Single crystal metal (Al(111)) Positron does not diffuse to surface and annihilates in the bulk e + emission Direct/thermal Ps emission

17 Single atom positronium decay in voids Single shot lifetime spectra measure the amount and the decay rate of o-ps. In porous materials the decay rate, γ, depends on the pore size (pick-off). Ps decays exponentially dn dt γ A < γ B γ Α = γ n γ Β

18 How do lifetime spectra change if Ps atoms interact with each other? If SEQ occurs, o-ps atoms are converted to p-ps atoms and decay rapidly (γ ~ 8 ns -1 ) If Ps 2 molecules are formed these will also decay rapidly (γ ~ 4 ns -1 ) Present resolution cannot tell the difference. Decay becomes non-linear: Ordinary (pick-off) decay dn dt decay due to Ps-Ps interactions = γ + β n( 1 n) parameter β describes strength of Ps-Ps interaction

19 10 0 Peak Normalised 10-1 dn n/dt β=0 β=1 β=2 γ = 0.02 ns -1 β=8 Time (ns)

20 10 β=8 Area normalized dn/dt 1 β=1 β= V = dn( t, β = 1) / dt dn( t, β = 0) / dt V Time (ns)

21 Simple analysis dn dt = γ n( 1+ βn) n = n0 ( 1+ βn )exp( γt) βn 0 0. Rate for Ps-Ps interaction : βγ = 2 n σv Obtain β and γ from fit of difference curve: V dn( t, β ) / dt dn( t,0) / dt = Aγ exp( γt) (1 + β )exp( γt) [(1 + β )exp( γt) β ] 2

22 Example of real data (I will show this in more detail tomorrow). Fitting difference curve gave rate, and then the cross section for Ps- Ps scattering. PMT Anode signal (V) System response Low density beam High density beam V (mev) High -low density fit Time (ns)

23 Can also look at transient events using low density Ps as a probe Paramagnetic centers can be created by UV light in many materials. Single shot Ps lifetime spectra can be used to measure the creation and lifetime of these defects because Ps decays faster due to the unpaired spins present (don t need high density beam for this)

24 Concluding remarks Intense pulses generate intense gamma signals. Fast and relatively inefficient detectors are needed to look at these signals. Single shot lifetimes can be used to observe interactions between Ps atoms The next lecture will explain some of the Ps-Ps interaction experiments we have done

25 Allen takes delivery of a gamma ray laser