Experimental production of manyExperimental production of many--

positron systems: L3, experimentspositron systems: L3, experiments

David B. Cassidy

Department of Physics and Astronomy,

University of California, Riverside, USA

cassidy@physics.ucr.edu Varenna, July 09

Overview• Experiments that need a lot of positrons (not trap

based experiments)• Early “Ps gas” experiments• Ps-Ps scattering• Ps formation• Ps2 formation• Radiation damage in silica• Ps Lyman alpha measurement• The (immediate) future….

Another Recap:Target chamber

Accumulator

Buncher

Source

Trap

0 100 200 300 400 500 600 700

1E-4

1E-3

0.01

0.1

1~ 40% Positronium (TEOS 1 kV)

< 2% Positronium (Phosphor screen, 5 kV)

norm

alis

ed d

etec

tor

outp

ut

time (ns)

•Surko trap used to make intense positron pulses

•Pulses implanted into Ps forming materials

•Ps lifetimes measured using single shot method

Positron channeling experiment at LLNL

(Alan Hunt et al)

Nature 401 157 (1999)(1999)

2.65 MeV positrons, 0.6 micron gold foil

Each data point took 12 hours!

Open circle SQA

Filled square TQA

Nuclear excitation by positron annihilation

PHYSICAL REVIEW C 64 054603

No signal was observed. Source based measurements of the cross section implied thatwe should have been able to see a signal.

What was the most intense beam What was the most intense beam In the world was run for 12 hours at 5 x 108 positrons/sec.

Perhaps this can now be done atMunich…….

What can happen when Ps atoms interact with each other?

• Nothing

o-Ps + o-Ps → o-Ps + o-Ps

• Spin exchange quenching (SEQ)• Spin exchange quenching (SEQ)

o-Ps(m=1) + o-Ps(m = -1)→ 2 p-Ps + 2Eho-Ps(m=1) + o-Ps(m = -1)→ 2 o-Ps(m=0)+ 2E h̀

• Positronium molecule (Ps2) formation

X+ o-Ps(m = 1) + o-Ps(m = -1)→ X+ Ps2 + Eb

First experiments showed Ps-Ps interactions, but we couldn’t

say what type.

PRL 95, 195006 (2005)

10-4

10-3

10-2

10-1 Randomly distributed Pores

PM

T A

node

sig

nal (

V)

System response Low density beam High density beam

0 50 100 150

0.0

0.5

1.0

1.5 High - Low density fit

∆V (

mV

)

time (ns)

Ps2

10-4

10-3

10-2

10-1Pores aligned in 1-D channels

PM

T A

node

sig

nal (

V)

System response Low density beam High density beam

0 50 100 150-1.0

-0.5

0.0

0.5

1.0

∆V (

meV

)

time (ns)

High -low density fitSEQ

How can we tell the difference

between SEQ and Ps2 formation?

• We can’t using only lifetime spectra; additional data are needed. PRL 95, 195006 (2005)

• SEQ requires that outgoing states accommodate • SEQ requires that outgoing states accommodate the hyperfine energy difference (~ 1 meV).

• Ps2 formation requires a third body (surface).

• → different sample properties help to distinguish between the two mechanisms

Two different porous silica films:

ψPores aligned along one dimension: continuum of accessible eigenstates

SEQ allowed ψ

ψ

SEQ allowed

Randomly distributed pores: only discrete eigenstates accessible.

SEQ suppressed

ψ

ψψ

20

Sample has a Ps surface state

Ps2 formation allowed

Indicates no Ps surface state

Ps2 formation suppressed

If there is a Ps surface state then heating the sample

will thermally desorb Ps, leading to an increase in the

Ps fraction

200 300 400 500 600

8

10

12

18

f d (%

)

Random pores Aligned pores Fit

Temperature (K)

Quenching data

1.0 1.5 2.0 2.5 3.0-0.4

-0.2

0.0

0.2

0.4

∆fd

(%)

n2D

(1010 cm-2)

180 K 384 K 517 K

∑−=∆m

DdDdDd nfm

nfnf1

222 )(1

)()(

Dd dnfdQ 2/∆≡

Ps-Ps interactions indicated by density dependent changes in lifetime spectra: the “quenching” effect

200 300 400 500

0

10

20

30 Quenching data Y(T) (scaled) Z

Z2

Q (

10-1

4 cm

-2)

Temperature (K)

))(1( TYZ −≡

Nature 449 196 (2007)

Form of Q(T) indicates that the Ps-Ps interactions occur via two surface state atoms

SEQ cross section

−++−−=−∝∆

2])exp()1[(

)exp()1()exp(/)0,(/),(

βγβγβγγβ

t

ttAdttdndttdnV

vn SEQσβγ 2=Rate for SEQ :

Obtain β and γ from fit of difference curve:

We find effective σSEQ≈ 9 ×10-15 cm2 PRL 100 013401 (2008)

Ivanov, Mitroy and Varga calculate σSEQ≈ 5 × 10-15 cm2 (E→ 0)

Phys. Rev. A 65 022704 (2002).

Difference is probably due to the unknown Ps thermalization rate

And uncertainties in the Ps density

Doing similar experiments with a metal target is very difficult on long timescales

Ps as a probe of paramagnetic centers in porous materials

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)

PHYSICAL REVIEW B 75, 085415 2007

Damage created everywhere in sample

No laser Ps fraction = 12%

If paramagnetic centers are present in the bulk the Ps fraction will be reduced

As before, Ps is created in the bulk material and then decays by “pick off” interactions with the internal pore surfaces.

If paramagnetic centers are present on the internal pore surfaces the o-Ps decay rate will be reduced

Thus Ps can distinguish between bulk and surface defects in these materials (ESR, OS cannot)

Fits = consistent with “fractal dynamics” theory

PRB 75 085415 (2007)

Tomu’s dye laser. Can be used for Ps 1S-2P (243 nm) of Ps2 (251 nm)

This has only just been set up, and we are just starting to take data with the laser. Ps 1S-2P transition is our first test of the system

I don’t know anything about lasers……..

laser fired

Single-shot lifetime spectra with the laser on and off:

Ps + hν (243) = Ps*

Ps* + hν(532) = e+ + e-

Target is a silica film from Laszlo Liszkay and co-workers

0 1 2 3 4 5 6 7 8 90

5

10

15

20

25

30 Sample phosphor screen

fd=W

2/W

1

W1 = -50-60 ns

W2 = 60-380 ns

f d (%

)

Beam Energy (keV)

0 100 200 300 400 500

0.01

0.1

80 100 120 140 160

Spectra averages of 10 shots

dete

ctor

out

put (

Vol

ts)

time (ns)

laser not fired

0.00

0.02

0.04

0.06

DV

/V λ = 242.95 nm λ = 243.03 nm

50 100 150 200

-0.02

0.00

time (ns)

After peak annihilation rate goes negative

preliminary data: only 1 week old

Natural line width ~ 50 MHz (due to 2P lifetime of 3 ns)

Ps is thermal so line width is dominated by the Doppler spread.

Width implies Ps energy is ~ 200 meV consistent with

0.6

0.8

1.0λ

0 = 242.953 nm

FWHM = 0.156 nm

no

rmal

ised

frac

tiona

l pea

k ar

ea

~ 200 meV consistent with Ps formation and cooling in porous films

More careful analysis needed (what is the correct Ps thermal distribution?)

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3

-0.2

0.0

0.2

0.4

norm

alis

ed fr

actio

nal p

eak

area

∆λ (nm)

The future (2009-2010)• Beam remoderation. Density increase initially to 5 ×1011 cm2 (pessimistic) and then later to 5 × 1012

(optimistic)

• Reliable Ps2 production on Al(111) surface

• Laser spectroscopy of Ps : Confirm existence of • Laser spectroscopy of Ps2: Confirm existence of molecule, perhaps measure lifetime of excited state

• Laser cool Ps, in vacuum and then in a cavity

• Make new larger cavities for PS BEC production

• Attempt to observe Ps BEC by laser spectroscopy (2010 just about possible if all goes well)

BEC transition temp

100

1000Ps BEC (after remoderation)

Ps-Ps interactionsSEQ, Ps

Crit

ical

tem

pera

ture

Tc (

K)

3/2)(1

nm

Tc ∝

1015 1016 1017 1018 1019 1020 10210.1

1

10

Ps laser cooling recoil limit

Stimulated annihilation(after multi-cell trap)

SEQ, Ps2

(present work)

Crit

ical

tem

pera

ture

T

Ps density (cm-3)

Long term future……Allen takes delivery of agamma ray laser (from first point scientific…)