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OUTLINE OF THIS TALKOUTLINE OF THIS TALKOUTLINE OF THIS TALKOUTLINE OF THIS TALK

Our studies for Ps－

- We started Ps－experiments in Tokyo Univ. of Sci. in 2005.
- Observation of Ps－ emission from tungsten surfaces

(Nagashima et al., New J. Phys.8 (2006) 319; Mat. Sci. Forum 607 (2009) 161)

- Efficient emission of Ps－ using Cs coated tungsten surface

(Nagashima et al., New J. Phys. 10 (2008) 123029; Phys. Status Solidi C 6 (2009) 2291)

- Emission of Ps－from Mo and Ta surfaces

(Michishio et al., J. Phys. Conf. Ser. 199 (2010) 012003)

- Durable emission of Ps－from Na coated tungsten surface

(Terabe et al., J. Phys. Conf. Ser. 262 (2011) 012058, also in preparation)

- Ps－photodetachment experiment

(Michishio et al., Phys. Rev. Lett. 106 (2011) 153401)

- Detection of Ps formed by the Ps－photodetachment

(Koji Michishio, O-19, on Wednesday)

OUTLINE OF THIS TALK

- What is Ps－?
- Efficient emission of Ps－ using Cs coated tungsten surface
- Emission of Ps－from Mo and Ta surfaces
- Durable emission of Ps－from Na coated tungsten surface
- Ps－photodetachment experiment
- Future plans
- Conclusions

OUTLINE OF THIS TALK

- What is Ps－?
- Efficient emission of Ps－ using Cs coated tungsten surface
- Emission of Ps－from Mo and Ta surfaces
- Durable emission of Ps－from Na coated tungsten surface
- Ps－photodetachment experiment
- Future plans
- Conclusions

Bound states composed of e+ and e－

Existence has been confirmed.

Existence has been reported.

(Cassidy and Mills, Nature 449 (2007) 06094)

Ps

Ps

Ps

e+

e－

＝

positronium molecule(Ps2)

positronium(Ps)

e－

e+

e－

positronium negative ion(Ps－)

Theoretical studies have been started.

(Frolov, Phys. Lett. A 372 (2008) 6721)

Ps

Ps

e－

bi-positronium negative ion(Ps2e－)

Nobody has produced.

Ps

Ps

e+

e+

e－

e+

bi-positronium positive ion(Ps2e＋)

positronium plus ion(Ps＋)

positronium negative ion(Ps－)

e－e+e－

- H atom like state
- The theoretical approach for H atoms can be applied.
- The wave function can be obtained
- without any approximations.

- H－ion like state
- All the constituents of Ps－
- have the same mass.
- The theoretical approach for H－
- (Born-Oppenheimer approximation)
- CANNOT be used for Ps－.
- Many theoretical approaches using variational principles have been performed.

positronium negative ion(Ps－)

e－e+e－

- Binding energy : 6.80eV
- Mean distance e+－e－: 2a0
- Two eigenstates
- ortho-Ps (S=1, triplet)
- lifetime in vacuum : 142ns
- Self-annihilates into 3γ .
- para-Ps (S=0, singlet)
- lifetime in vacuum : 125ps
- Self-annihilates into 2γ .

- e－binding energy to Ps = 0.33eV;
- Total binding energy (the energy
- required to break up Ps－ into 3
- isolated particles) :
- 0.33eV + 6.80eV = 7.13eV
- Mean distancee+－e－: 5.5a0
- Only one state
- Lifetime in vacuum : 479ps
- Self-annihilates into 2γ.

e－e+e－

History of Ps－research

1946J.A. Wheeler predicted the existence.

Many theoretical researches have been performed since the prediction of Wheeler.

Examples of theoretical investigations on Ps－ :

1960 Calculation of the Ps- binding energy byKolos, Rootrhaan and Sack.

1964 Calculation of the Ps- binding energy by Frost, Inokuti and Lowe.

1968 Calculation of the Ps- decay rate by Ferrante.

1979 Calculation of the Ps- binding energy by Ho.

1983 Calculation of the Ps- binding energy and decay rate

by Bhatia andDrachman.

1985 Calculation of Ps- photodetachment cross sections by Bhatia and Drachman.

1987 Calculation of Ps- photodetachment cross sections by Ward, Humberstonand McDowell.

1990 Calculation of the Ps- decay rate by Ho.

1993 Calculation of Ps- binding energy by Ho.

2000 Calculation of the Ps- binding energy by Korobov.

2000 Calculation of Ps- photodetachment cross sections by Igarashi, Shimamura and Toshima.

2002 Calculation of Ps- binding energy by Drake, Grigorescu and Nistor.

2005 Calculation of the Ps- binding energy by Drake and Grigorescu.

EB＝ 0.261 998 108 122 au ~ 7.129 330 97 eV

2007 Calculation of the Ps- decay rate by Puchalski and Czanecki.

Γ= 2.087 963 (12) ns-1.

e－e+e－

History of Ps－research

1946J.A. Wheeler predicted the existence.

1981A. P. Mills, Jr. succeeded in the production.

(formation efficiency=0.028%)

Ps－formation efficiency＝number of formed Ps－/number of incident slow e+

First observation of Ps－(Mills, 1981)

- Slow positrons (470 eV) were guided to a thin carbon target.
- Ps－emittedwere accelerated by the electric field and detected by their Doppler-shifted annihilation lines.

θ

Ps－formation was confirmed.

Ps－formationefficiency = 0.028%

(A.P. Mills, Jr., Phys. Rev. Lett. 46 (1981) 717)

Measurement of the Ps－decay rate

Tandem acceleration method of detecting Ps－

A. P. Mills, Jr., P. G. Freeman and D. M. Zuckerman, NASA Conference Publication (1989)

Measurement of the Ps－decay rate

Ps- fraction = 1 x 10-4

（30eV）

Stripping-based

detection technique

(Fleischer et al., Phys. Rev. Lett. 96 (2006) 063401)

e－e+e－

History of Ps－research

1946J.A. Wheeler predicted the existence.

1981A. P. Mills, Jr. succeeded in the production.

(formation efficiency=0.028%)

Many theoretical researches have been performed.

Only a few experiments have been performed.

Ps－formation efficiency is too low.

OUTLINE OF THIS TALK

- What is Ps－?
- Efficient emission of Ps－ using Cs coated tungsten surface
- Emission of Ps－from Mo and Ta surfaces
- Durable emission of Ps－from Na coated tungsten surface
- Ps－photodetachment experiment
- Future plans
- Conclusions

When e+ are incident onto metal surfaces, ...

Diffusion length of e+ in defect free metals

~ 100nm

annihilation cross section

<< collision cross section

e+ lifetime in metals ~ 100ps

A significant fraction of e+

incident onto surface with a few keV energy

diffuse back to the surface.

The energy required to emit Ps :

: e+work function

: e－ work function

Ps atoms are emitted

from the surface.

The energy required for Ps－emission :

e－work function

e+ work function

Ps－binding energy

(The energy required to break up

Ps－into three isolated particles)

Ps－might be emitted

from the surface spontaneously.

- tungsten (1 0 0)
- tungsten (1 1 1)
- polycrystalline tungsten

Ps－ions may be emitted.

- tungsten (1 0 0)
- tungsten (1 1 1)
- polycrystalline tungsten

Ps－ions may be emitted.

Ps－emission from polycrystalline tungsten surface

(Nagashima and Sakai, New J. Phys. 8 (2006) 319)

vacuum : 7 x 10-8 Pa (5 x 10-10torr)

Ps－formation efficiency was only 0.007%.

(1/4 of that of beam-foil method)

Ps－emission from polycrystalline tungsten surface

(Nagashima and Sakai, New J. Phys. 8 (2006) 319)

Ps－formation efficiency decreases.

Change of the surface condition

D

e－energy levele+energy lebel

(Achcroftand Mermin) (Schultz and Lynn, Rev. Mod. Phys. 60 (1988) 701)

e－ work function :

e+work function :

：e－ chemical potential

：e+ chemical potential

：effect of surface dipole

Ps－emission from polycrystalline tungsten surface

D becomes larger

by the effect of oxygen, H, H2O, .....

Ps－formation efficiency decreases.

Ps－emission from polycrystalline tungsten surface

Time dependence of Ps- formation efficiency

7 x 10-8 Pa

3 x 10-8 Pa

When the vacuum was improved, the fraction became stable.

Ps－emission from polycrystalline tungsten surface

Time dependence of Ps- formation efficiency

7 x 10-8 Pa

3 x 10-8 Pa

When the vacuum was improved, the fraction became stable.

The dependence was due to the adsorbate coverage of the target surface

by residual molecules in the target chamber.

(Nagashima and Sakai, New J. Phys. 8 (2006) 319,

Nagashima, Hakodate and Sakai, Appl. Surf. Sci. 255 (2008) 217)

Effect of Cs coating for the Ps－emission

Change of for tungsten by Cs coating

D decreases.

decreases.

Ps－ formation efficiency might increase.

Kiejna and Wojciechowski,

Prog. in Surf. Sci. 11 (1981) 293

Effect of Cs coating for the Ps－ emission

e+transport energy ：100eV

e+ incident energy onto the target : 100eV + eW

TargetW(100)

vacuum：2×10-8Pa

（1.5×10-10torr）

Target was annealed

at 1500℃ for 30 min.

target potential ：-3kV

(Nagashima et al. New J. Phys. 10 (2008) 123029)

Effect of Cs coating for the Ps－ emission

(Nagashima et al. New J. Phys. 10 (2008) 123029)

Effect of Cs coating for the Ps－ emission

Ps－intensity is the highest

at 2.2×1014cm-2（0.8ML).

Change of for tungsten by Cs coating

Kiejna and Wojciechowski, Prog. in Surf. Sci. 11 (1981) 293

Effect of Cs coating for the Ps－ emission

The highest efficiency was

1.25%,

which is two orders of magnitude higher

than that obtained for uncoated surface,

and 45 times greater

than the beam-foil method.

- What is Ps－?
- Efficient emission of Ps－ using Cs coated tungsten surface
- Emission of Ps－from Mo and Ta surfaces
- Durable emission of Ps－from Na coated tungsten surface
- Ps－photodetachment experiment
- Future plans
- Conclusions

Ps－emission from Cs coated Mo surfaces

2 x 1014 atoms/cm2

Ps－ions were detected for uncoated Mo

and Cs coated Mo.

Ps－emission from Cs coated Mo surfaces

2 x 1014 atoms/cm2

Ps－ions were detected for uncoated Mo

and Cs coated Mo.

First experimental evaluation for EB

Ps－emission from Cs coated Mo surfaces

2 x 1014 atoms/cm2

Ps－ions were detected for uncoated Mo

and Cs coated Mo.

for the uncoated Mo surface.

：Ps- binding energy

Ps－emission from Cs deposited Ta surfaces

2 x 1014 atoms/cm2

Ps－ions were not detected for uncoated Ta,

but

Ps－ions were detected for the Cs coated Ta.

The efficiency was 1.5%,

which is higher than that for the Cs coated

W(100).

Ps－emission from Cs deposited Ta surfaces

2 x 1014 atoms/cm2

Ps－ions were not detected for uncoated Ta.

for the uncoated Ta surface.

：Ps－binding energy

Ps－emission from Cs deposited Ta surfaces

2 x 1014 atoms/cm2

Ps－ions were not detected for uncoated Ta.

for the uncoated Ta surface.

：Ps－binding energy

- What is Ps－?
- Efficient emission of Ps－ using Cs coated tungsten surface
- Emission of Ps－from Mo and Ta surfaces
- Durable emission of Ps－from Na coated tungsten surface
- Ps－photodetachment experiment
- Future plans
- Conclusions

Effect of Cs coating for the Ps－ emission

carbon film

Time dependence of f

The decrease might be due to

the accumulation of residual molecules.

Effect of K and Na coating for the Ps－ emission

K and Na are less reactive chemically than Cs.

Ps－emission might stay longer.

Cs K Na

The effect might be smaller.

Kiejna and Wojciechowski, Prog. in Surf. Sci. 11 (1981) 293

Effect of K and Na coating for the Ps－ emission

Na coating is as effective

and the effect remains LONGER!

- What is Ps－?
- Efficient emission of Ps－ using Cs coated tungsten surface
- Emission of Ps－from Mo and Ta surfaces
- Durable emission of Ps－from Na coated tungsten surface
- Ps－photodetachment experiment
- Future plans
- Conclusions

Theoretical investigations for the photodetachment of Ps－ :

Calculation of the photodetachment cross sections

by Bhatia and Drachman

1987Calculation of the photodetachment cross sections

by Ward, Humberston and McDowell

2000 Calculation of the photodetachment cross sections

by Igarashi, Shimamura and Toshima

（Igarashi et al, New J. Phys. 2 (2000) 17）

Ps－lifetime is 479ps.

High intensity pulsed laser

Pulsed e+ beam

synchronized to the laser

http://www-linac.kek.jp/slowpos/OVERVIEW.PDF

e+ beam :

(from KEK Linac)

pulse width 12ns

repetition 50Hz

Laser :

Pulsed Nd: YAG

(Spectra Physics GCR290)

wave length 1064nm

pulse width 12ns

repetition 25Hz

power 10W

(Michishio, Tachibana, Terabe, Igarashi, Wada, Hyodo, Kuga, Yagishita, Hyodo and Nagashima

Phys. Rev. Lett. 106 (2011) 153401)

1064nm

Threshold : 0.33eV

(Bhatia and Drachman, Phys. Rev. A 32 (1985) 3745,

Ward, Humberston and McDowell, J. Phys. B 20 (1987) 127,

Igarashi, Shimamura and Toshima, New J. Phys. 2 (2000) 17)

e+ beam :

(from KEK Linac)

pulse width 12ns

repetition 50Hz

Laser :

Pulsed Nd: YAG

(Spectra Physics GCR290)

wave length 1064nm

pulse width 12ns

repetition 25Hz

power 10W

(Michishio, Tachibana, Terabe, Igarashi, Wada, Hyodo, Kuga, Yagishita, Hyodo and Nagashima

Phys. Rev. Lett. 106 (2011) 153401)

75% o-Ps, annihilates into 3γ.

25% p-Ps, annihilates into 2γ.

Ps－

If Ps－ions are photodetached,

the peak intensity will decrease.

(Michishio, Tachibana, Terabe, Igarashi, Wada, Hyodo, Kuga, Yagishita, Hyodo and Nagashima

Phys. Rev. Lett. 106 (2011) 153401)

75% o-Ps, annihilates into 3γ.

25% p-Ps, annihilates into 2γ.

Ps－

If Ps－ions are photodetached,

the peak intensity will decrease.

Ps－photodetachment has been observed for the first time!

(Michishio, Tachibana, Terabe, Igarashi, Wada, Hyodo, Kuga, Yagishita, Hyodo and Nagashima

Phys. Rev. Lett. 106 (2011) 153401)

(Michishio, Tachibana, Terabe, Igarashi, Wada, Hyodo, Kuga, Yagishita, Hyodo and Nagashima

Phys. Rev. Lett. 106 (2011) 153401)

- What is Ps－?
- Efficient emission of Ps－ using Cs coated tungsten surface
- Emission of Ps－from Mo and Ta surfaces
- Durable emission of Ps－from Na coated tungsten surface
- Ps－photodetachment experiment
- Future plans
- Conclusions

OUTLINE OF THIS TALK

- What is Ps－?
- Efficient emission of Ps－ using Cs coated tungsten surface
- Emission of Ps－from Mo and Ta surfaces
- Durable emission of Ps－from Na coated tungsten surface
- Ps－photodetachment experiment
- Future plans
- Conclusions

- Measurement of the photodetachment cross sections
- Production of energy tunable Ps beam
- Precision measurement of the Ps－decay rate

Measurement of the Ps－photodetachment cross sections

(Michishio, Tachibana, Terabe, Igarashi, Wada, Hyodo, Kuga, Yagishita, Hyodo and Nagashima

Phys. Rev. Lett. 106 (2011) 153401)

Measurement of the Ps－photodetachment cross sections

resonances

（Igarashi et al, New J. Phys. 2 (2000) 17）

Production of energy tunable Psbeam using Ps－photodetachment technique and its applications

“Production of an energy tunable positronium beam

by the photodetachment of positronium negative ions”

by Koji Michishio on Wednesday

Measurement of the Ps－decay rate

Ps- fraction = 1 x 10-4

（30eV）

Stripping-based

detection technique

(Fleischer et al., Phys. Rev. Lett. 96 (2006) 063401)

Precision measurement of the Ps－decay rate

Ne dispenser

By changing the distance and voltage between the target and the earthed grid,

we aim high precision measurementwith the error ～ 0.1%

(1)We have succeeded in the efficient formation Ps－.

(2) We have succeeded in the first estimation of the binding energy of Ps－.

(3) We have succeeded in first observation of the photodetachment of Ps－.

(4) We have started the next experiments for Ps－.

Member of our group

Tokyo University of Science

- Koji Michishio, Takayuki Tachibana

Hiroki Terabe, Ryohei Suzuki

Ayaka Miyamoto, Toshihide Hakodate

Takahiko Sakai

- Akira Yagishita (KEK)

Toshio Hyodo(KEK)

Ken Wada (KEK)

Akinori Igarashi (Miyazaki Univ.)

Takahiro Kuga (Univ. of Tokyo)

Why Ps-fraction increased dramatically?

If we assume that the Ps- is formed from positrons in the bottom of the positron band,

holds for the Ps- emission,

where and are the energy levels of two electrons measured from the vacuum level.

Ps- fraction :

: electron density of states

If is constant below

the top of the conduction band,

This is too low to explain the experimental results.

Why Ps-fraction increased dramatically?

The Ps-formation depends on the overlap of a e+ wave function

and two e- wave functions just outside of the surface,

which depend on their corresponding work functions.

The Ps- formation mechanism

should be affected

by the Cs deposition.

Cs deposited

uncoated

e+ and e- wave functions

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