Protonium spectroscopy

90

Nuclear Physics B (Proc Suppl ) 8 (1989) 90-99 North-Holland, Amsterdam

PROTONIUM SPECTROSCOPY ROBERT E. WELSH Department of Physics, The Colleg~ of William and Mary, Williamsburg, Virginia 23185, USAn Protonium Experiments at CERN-LEAR are described and their results compared with theoretical predictions. The data are mutually complementary and form a s t a t i s t i c a l l y consistent set. Information is extracted from these data about the cascade of hadronic hydrogen atoms and about the antiprotonproton interaction at threshold. 1. INTRODUCTION The f i r s t experiments designed to measure the x-rays from the ~p atomic system were installed at CERNI,2"and at Brookhaven3 in the 1970's. These experiments used ~ beams of modest intensity, poor momentum resolution and large beam-spot size.

None of these early experiments detected the K x-rays

of protonium but the experiment of Auld, et al. 2 was successful in observing the L x-rays with a large cylindrical proportional chamber array that surrounded a four atmosphere gaseous H2 target.

There are several reviews4 which

provide details not covered here. RECENT WORKAT CERN-LEAR With the a v a i l a b i l i t y of ~ beams of excellent quality at the CERN-LEAR machine, three experiments, PS171, PS174 and PS175 were conceived to study the atomic x-rays from protonium. These experiments were complementary and their data provide a far more comprehensive understanding of the ~p atomic system than was previously available.

Experiment PS171, the Asterix experiment5,

used a central cylindrical gaseous target that could be f i l l e d with H2 or D2 at I atm.

I t was surrounded by a gO-cell x-ray d r i f t chamber (XDC) and then

by seven concentric proportional "chambers for detection of the charged annihilation products.

Also, v-ray detection was achieved by the use of Pb convert-

er, between MWPCs 5 and 6.

Although the Asterix x-ray detector had poorer

resolution than the solid state detectors and gas s c i n t i l l a t i o n proportional detectors (GSPD) used in the other two protonium experiments at LEAR, i t had nearly 4~ solid angle. This feature permitted the simultaneous detection of K and L x-rays from the protonium atom. Additionally, the surrounding MWPCs *Work supported by the U. S. National Science Foundation. 0920-5632/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

R.E. Welsh/Protonium spectroscopy

91

could detect the products of annihilation in coincidence with the x-rays. This permitted identification of final state products associated with annihilation from specific atomic states 6. Experiment PS174 7 consisted of a cylindrical, high purity Al target flask viewed from above by a Si(Li) detector 300 mm2 by 5 mm thick.

The target was

surrounded by an outer vacuum jacket and a helium refrigerator was used for operation at 30K (P = IOPSTP) and 150K (P = 2PSTP). For operation at ambient temperature, the vacuum jacket was removed and two GSPDswere incorporated into the side walls of the target flask.

These GSPDs, described in detail

elsewhere8, have larger solid angle and lower s e n s i t i v i t y to Comptonoscattered v-ray background than Si(Li) detectors.

The GSPDs, on the other hand, had

resolution a factor of two poorer than the Si(Li) but about three times better than the XDCs and provided an independent detection technique. The experimental apparatus of experiment PS175, the Inverse Cyclotron Trap, is described in detail elsewhere9.

A superconducting magnet maintained a

field of 3-4T within the cyclotron trap.

Antiprotons were directed into the

device through a thin moderator which caused them to be trapped in the magnetic f i e l d .

They then followed a spiral path while being slowed by the

gas in the target region, f i n a l l y stopping in the center of the device.

One

or more solid state detectors viewed the central region through openings in the magnet pole piece.

Scintillators that lined the inner faces of the pole

pieces served to reduce background which could originate from interactions of which scattered into the poles while being moderated. The cyclotron trap has been operated at gas densities from PSTP to 0.03 PSTP. I t is quite useful at low pressures since, in contrast to the stopping distribution typical of very low density targets, about 40% of injected ~ come to rest in the central region at a pressure of 60 mbar.

I t is a tribute to the designers of LEAR,

however, that, as reported by PS174, the quality of LEAR beams (Ap/p = 10-3 ) permitted more than 80% of the 105 MeV/c ~ beam to be brought to rest in H2 at 0.25 PSTP" By contrast, in the earlier BNL experiment with a 620 MeV/c beam (Ap/p = ±1%), only about 3% of the ~ in the beam came to rest in a similar gaseous target 3.

The d i s t i n c t advantages of LEAR thus become obvious.

In the

older, high-momentum beams, thick moderators were necessary to slow the beam. Such moderators became both a source of background and a cause of spreading of the beam in space and momentum. Beam was f i r s t delivered to the LEAR experiments in 1983 at 308 MeV/c. At that momentum the stopping ~ beam has a distribution of 25mg/cm2 FWHMand thus 70% of the beam could be contained within 30 cm for the PS174 H2 gas target cooled to 30K at atmospheric pressure (10% for H2 gas at STP). As the LEAR

R.E. Welsh/Protonium spectroscopy

92

extraction momentum was reduced to 202 MeV/c and then to 105 MeV/c the data quality from these experiments improved. E (keV)

I n

- 1.4

0

;

1

2

3

_

_

.. __Xf,o7 T3p

" $ ///~

-3.1

3d

ELECTROMAGNETIC ENERGIES (keV) :C

2p ~

K~ = 9.4 K~ -- 11.1 Koo-- 12.5 L(X i. 7 L~ 2.3 Loo = 3 . 1

1

E'ls

]~ls

,,ii

Fig. I

Lowest atomic states of protonium.

CALIBRATION Each of the three experiments described above has taken data in gaseous H2, D2, and He over an x-ray energy range from about I keV to 25 keV. The He gas data provide important checks of the apparatus including energy calibration under beam conditions, since ~ He atomic x-rays range in energy from less than 2 keV to 16 keV. Thus, important effects such as target and window absorption, electronic timing efficiency and detector behavior could regularly be checked and monitored.

The LEAR ~ beam was c h a r a c t e r i s t i c a l l y delivered in

" s p i l l s " lasting about an hour.

The time required to f i l l

the LEAR machine

and cool and prepare the beam for extraction was about 15 minutes, adequate for calibration runs or target gas or pressure changes. Each of the experiments could introduce radioactive sources for calibration. I t must be emphasized that the data taken in ~d are both of i n t r i n s i c interest and provide an independent check for possible background in the ~p K region. ANTIPROTONIC HELIUM In addition to the usefulness of ~-He data for calibration of the apparatus, the strong interaction width of the ~-He 2p and 3d atomic states have been measured using these techniques. IO ANTIPROTONIC HYDROGEN The combined work of PS171, PS174,and PS175 has yielded results on protonium at densities ranging from 10 PSTP to 0.03 PSTP. The general characteristics of the ~p atomic system are demonstrated by the cascade

R.E. Welsh/Protonium spectroscopy diagram shown in Fig. ].

93

One assumes that the ~p system becomes bound in an

atomic state of principal quantum number n ; 30 or higher.

Transitions to

lower states then occur via Auger, chemical, radiative and Stark-mixing interactions.

Annihilation occurs rapidly from s-states and thus the

a

50 000

b

ASTERIX (PS 171) MAY 86 Data

60 000

/

L

Lines

000

series

S00

4,0 000

K Lines series

/

L~. Line

~

200

30 000 S00

20 000 L,00

10 000 0

K= Lone

i

i

5

i

10

X - RAY ENERGY [KeV|

a) all events,

0

i

15

20

5

10

15

20

X - RAY ENERGY [ K E Y ]

Fig. 2 PS171 protonium data. b) spectrum of events with coincident x-rays.

collision frequency, proportional to the target density, has considerable effect on the atomic cascade. Those protonium atoms which survive capture to reach the 2p state have approximately I% probability to radiate to the Is state as the 2p radiative width is 0.38 meV and the annihilation width from this state is about 40 meV.12 Each transition to the Is state will exhibit the effects of the strong interaction with the line being shifted and broadened around i keV. The K x-ray transitions thus provided the major common goal of the three experiments discussed here. RESULTS The earliest published protonium measurements at LEAR resulted principally from data taken in 1983 at the i n i t i a l LEAR beam momentum of 308 MeV/c.11,12 Since that time, reduction of LEAR beam momentum and improvements in the individual experiments have yielded data of improved quality and consistency. PS171ASTERIX EXPERIMENT The x-ray data f i r s t reported by PS 171 based on beam taken at LEAR in 1983 p a r t i a l l y exploited the capabilities of their apparatus. 11 These data were taken with the surrounding MWPCsin anticoincidence to suppress bremsstrahlung from charged annihilation products, but data were not reported for measurement of coincident x-rays.

More recently, this collaboration has reported 13 x-ray

data taken at 105 MeV/c which required a coincidence between L x-rays and K x-

94

R.E. Welsh / Protonium spectroscopy

rays thereby improving the quality of these measurements. These data show a Ko l i n e essentially free of background. A comparison of data in the 10 keV region accumulated with and without the requirement of a coincident L l i n e , Fig. 2, attests to the usefulness of the 4= x-ray detector.

This requirement

permits the observation of a single K x-ray t r a n s i t i o n , the 2p~is, to the exclusion of the other K lines and with essentially no background. More recently, PS 171 has reported the branching ratios for ~p annihilation at rest into x+=" or K+K- from "pure"

~ = I states. 6 These experiments required

coincident observation of a Balmer series x-ray which would exclude all but about i% s-state capture.

By combining these data with e a r l i e r work in l i q u i d

hydrogen one determines a suppression factor of four for ~p~K+K- from states.

~= I

The data also indicate 50% p-wave annihilation for ~p in H2 gas at

STP.

60~ F

I

, j;~ '

Antlprotomc Hydrogen K X-rays

~

z.

L~

z c] L ~

200!

f

"-

E

Fig. 3.

8

]E~

1~ ~ X-RAY ENERGY (keY)

l~_

"8

.,-->

Protonium x-rays taken with GSPD.

PS 174 EXPERIMENT In contrast to PS 171, this experiment has been operated at H2 and D2 gas densities as low as 0.25 PSTP and as high as 10 PSTP. While lacking the a b i l i t y to observe coincident x-rays in the cascade, the a b i l i t y to carry out protonium measurements at a variety of densities proved to be very usef u l . 12'14 Considerable data in D2 were taken for comparison with the H2 data. The e a r l i e s t results from PS 174 were of ~ stopping in He gas. 10 From these data the strong interaction s h i f t and width were extracted for the ~ He atomic 2p state and the width of the 3d state.

Since then, the experiment has

yielded protonium measurements in H2 gas measured both with Si(Li) detectors and, more recently, with GSPDs.]5

In addition, this experiment has obtained

R.E. Welsh/Protonium spectroscopy

95

further measurements in ~He.16 Data at PSTP and below were taken at room temperature with Si(Li) and GSPDsimultaneously. taken at 0.92 PSTP with GSPDare shown in Fig. 3.

Antiprotonic hydrogen data In Fig. 4 protonium data

taken over a density range from d.25 to 10 PSTP using Si(Li) are shown. Figure 4 shows the variation in relative yields among the protonium K lines as the gas density is varied. Stark mixing.

This variation arises mainly from the effects of

This process serves to enhance capture from higher n,

~= 0

states and thus to reduce absolute K line yields at higher densities.

In

addition, the distribution of populations is shifted to enhance "inner" transitions to the lower n states.

The result then is to reduce the yield

ratio Ke to Ktota I . PS 175 EXPERIMENT, THE INVERSECYCLOTRONTRAP PS 175 have reported data17 from ~H, ~D and ~He at densities as low as 0.016 PSTP (see Fig. 7 of Ref. 17).

The relative yield of the K lines

corresponds well to cascade predictions and the results obtained at a variety of higher pressures by PS 174. The PS 175 group have made several other interesting measurements in gaseous targets using their trap.

Among these are

measurements 17 in antiprotonic Ar and Kr which demonstrate that at pressures of the order of 25-50 mbar these'atoms are highly ionized by the Auger process and are not r e f i l l e d in times corresponding to the radiative transition periods in the atom. This collaboration may be able to measure the individual protonium IS hyperfine states in future runs at LEAR. CASCADE CALCULATIONS The protonium cascade has been studied by a number of workers.

The

calculations of Leon and Bethe, 18 Borie and Leon19 and similar extensions of the cascade program of H~fner20 have been used with success in the interpretation of the data of PS 174 and PS 175.

In these calculations one assumes a

Stark-mixing parameter and a protonium kinetic energy (generally chosen to be ! eV) to include the influence of the Stark effect on the protonium cascade. More recently Reifenr6ther and Klempt21 have presented a cascade calculation, similar to earlier work by Landua and Klempt,22 that uses a classical trajectory Monte Carlo method to determine the Stark-mixing parameter from f i r s t principles (an i n i t i a l protonium kinetic energy of I eV is chosen). This also results in reasonably good f i t s to the data from the three experiments.

It

should be stressed that the L x-ray data provide quite useful results for testing the cascade calculations.

The L-line intensities have been measured

for both ~H and ~D at a variety of pressures and for the most part those data are mutually consistent. One point worth noting is the determination of the 3d width in ~D. The data 17 from PS 175 imply a strong annihilation width of

R E Welsh /Protonium ~pectro~rot)y

96

+17 F3d = 50 -11 peV. Similar data 14'15 from PS 174 at higher pressures can be f i t reasonably well with a 3d strong width of about 5 peV which is in better agreement with theory32 but are f i t s l i g h t l y better with the larger width. Should subsequent experimental data verify the larger width, rather drastic modifications of the theory would be indicated. STRONG INTERACTION EFFECTS The principal strong interaction results which can be extracted from these experiments are the width and s h i f t of the Is atomic state.

Although informa-

tion on the individual Is hyperfine states would be of considerable interest, none of the present experiments can distinguish those states and so one discusses the results in terms of a mean s h i f t and width.

Each of the experi-

ments has made a determination of these parameters. The Si(Li) data with excellent resolution are complemented by the XDC results of PS 171 in which the Ks l i n e is seen with almost no background albeit with poorer resolution. One should also stress the importance of the L-line data and cascade Energy Shift AEIs

(keV)

Width F1s (keV)

Re as (fm)

Im as (fm)

EXPERIMENT van Eijk et a115(PSI74) -0.73±0.05

1.13±0.09 -0.935±0.081 0.874±0.073

Baker et a114(ps]74)

-0.75±0.06

0.09±0.18

Ahmad et al11(PS171)

-0.50±0.30

<1.0

Ziegler et al13(PS171)

-0.70±0.15

1.60±0.40

Bacher et a117(PS175)

-0.66±0.13

1.13±0.23

Bryan & P h i l l i p s 23

-0.87

1.31

-i.000

0.751

Richard & Sainio 24

-0.71

0.93

-0.923

0.739

-0.923

0.739

-0.979

0.580

-1.030

0.790

THEORY

-0 75

0.95

Green & Wycech25

-0 80

1.28

Moussallam26

-0 75

1.01

Schweiger et a127

-0 85

1.0

Kaufmann28

-0 69

I .09

Thaler29

-0 86

1.14

Alberg et al30(best f i t )

-0 94

1.4

Table I.

ProtoniumStrong-lnteraction Effects

R.E. Welsh/Protonium spectroscopy

97

calculations in permitting a de-convolution of the family of broadened and shifted K lines detected by PS 174 and PS 175.

In Table I one can see the

strong-interaction data extracted from the LEAR experiments and for comparison several theoretical predictions.

As the two ~p 1S hyperfine states are not

resolved experimentally, the quoted theoretical values are averaged over the two spin states.

The experimental results can be seen to be mutually consis-

tent and in agreement with the range of potential models in the l i t e r a t u r e . The so-called p parameter, defined as

t,1,

300 ,

200E

z

11~, [

I 10,• PSTP

I

230

l ~ . l o.92ps,~

Tf"llT ~r~-~.~--~,,, j

I 8~

l

~~ l

,

~,,,,,i~I~

28°

.~-'~

2RC

180 130

i ............

6

,.

.

.

8

.

.

.

.

.

10

[ ~F~L

Fig. 4

txr

.

T

.

.

I~

.

.

.

.

14

.

.

.

~C

;8

keV'~

Protonium K-x ray data taken at a range of H2 pressures.

the r a t i o of real to imaginary part of the forward scattering amplitude, is often compared15 to the result extracted from low energy pp scattering experiments.

For a discussion of t h i s comparison and for a more complete

description of the potential models used in the theoretical predictions the reader is referred to ref. 15 and other work cited therein. SUMMARY The most recent protonium results from LEAR are in good mutual agreement and are compatible with most theoretical predictions.

The surprising widths

and s h i f t s suggested by e a r l i e r workI have not been seen at LEAR and thus the

R.E. Welsh / Protonium spectroscopy

98

later data do not appear to require the existence of baryonium states near threshold. 31 As existing data cannot unfold the individual n = i protonium hyperfine states there is a clear need for more precise measurements. Yield determinations in ~d by PS174 and PS175 can be f i t better by an anomalously large 3-d capture width in contrast to theoretical predictions. 32 ACKNOWLEDGEMENTS I am grateful for the advice and assistance of members of the PS174 collaboration and to C. J. Batty, J. D. Davies, M. Eckhause and R. G. Winter for a careful reading of the manuscript.

I am indebted to M. Alberg, C.

Dover, U. Gastaldi, E. Klempt, R. Landua, and L. Simons for communicating their results and for helpful suggestions.

Thanks for preparation of the

manuscript are due D. Fannin and S. Stout. N. Tanner (Oxford): ments?

Are you confident of the various width and shift measure-

R. E. Welsh: Yes, they agree well s t a t i s t i c a l l y . A. M. Green (Helsinki): width?

Should theorists be concerned about the ~d, 3-d

R. E. Welsh: Probably not until further measurements corroborate this. E. Klempt (Mainz): I would like to stress that the cascade results carried out here are not f i t s but calculations. REFERENCES I.

M. Izycki, et al., Z. Phys. A297 (1980) I.

2.

E. G. Auld, H. Averdung, et al., Phys. Lett. 77B (1978) 454.

3.

J. R. Lindemuth, M. Eckhause, et al., Phys. Rev. C30 (1984) 1740.

4.

C. J. Batty, Nucl. Phys. A372 (1981) 433; and, Proceedings of the International School of Physics with Low Energy Antiprotons, Erice, June, 1988; E. Klempt, Antiprotonic Hydrogen, in "The Hydrogen Atom", PISA, (1988) ed F. Bassani, Springer Verlag, Berlin and "Atomic Physics with Antiprotons", Thessaloniki, (1986); H. Poth, Antiprotonic, Hyperonic and Antihydrogen Atoms, CERN-EP/86-105.

5.

U. Gastaldi, S. Ahmad, et al., Phys. at LEAR, eds. U. Gastaldi and R. Klapisch (Plenum, New York, ]984) p. 109.

6.

M. Doser, M. Botlo, et al., Nucl. Phys. A486 (1988).

7.

J. D. Davies, Physics at LEAR, eds. U. Gastaldi and R. Klapisch, (Plenum, New York, 1984) p. 319.

8.

W. J. C. Okx, C. W. E. van Eijk, et al., IEEE Trans. Nucl. Sci. NS33 (1986) 391, and Nucl. Inst. Meth. A252 (1986) 605.

R.E. Welsh/Protonium spectroscopy 9.

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L. Simons, Physica Scripta T22 (1988) 89.

10.

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S. Ahmad, C. Amsler, et al., Phys. Lett. 157B (1985) 333.

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M. Ziegler, S. Ahmad, et al., Phys. Lett. 206B (1988) 151.

14.

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15.

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16.

C. A. Baker, C. J. Batty, et al., to be published.

17.

L. Simons, Proc. IV LEAR Workshop, C. Amsler, et al., Eds., Harwood Acad. Publ., Villars (1987) 703. and, R. Bacher, P. Blum et al., to be published.

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20.

J. HUfner, private communication.

21.

G. Reifenr6ther and E. Klempt, Phys. Lett., to be published.

22.

R. Landua and E. Klempt, Phys. Rev. Lett. 48 (1982) 1722. See also G. Reifenr6ther, E. Klempt and R. Landua, Phys. Lett. B191 (1987) 15, B203 (1988) 9; also G. Reifenrother, this volume.

23.

R. A. Bryan and R. J. N. Phillips. (1968) 481.

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J. M. Richard and M. E. Sainio.

25.

A. M. Green and S. Wycech. Nucl. Phys. A377 (1982) 441.

26.

B. Moussallam. Z. Phys. A325 (1986) I.

27.

W. Schweiger, et al., Phys. Rev. C32 (1985) 1261.

28.

W. B. Kaufmann. Phys. Rev. C19 (1979) 440.

29.

J. Thaler, J. Phys. G:

30.

M. A. Alberg, et al., Phys. Rev. D27 (1983) 536.

31.

I. S. Shapiro, Phys. Reports 35C (1978) 149.

32.

S. Wycech, A. M. Green, and J. A. Niskanen, Phys. Lett. B152 (1985) 308.

see also E. Borie and

Nucl. Phys. B5 (1968) 201 and B7

Phys. Lett. 110B (1982) 349.

Nucl. Phys. 9 (1983) 1009.