NUCLEAR POLARIZATION MEASUREMENTS OF HYDROGEN ATOMS FROM STORAGE CELLS

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NUCLEAR POLARIZATION MEASUREMENTS OF HYDROGEN ATOMS FROM STORAGE CELLS

W. Luck, H. Jänsch, D. Fick, E. Steffens

To cite this version:

W. Luck, H. Jänsch, D. Fick, E. Steffens. NUCLEAR POLARIZATION MEASUREMENTS OF

HYDROGEN ATOMS FROM STORAGE CELLS. Journal de Physique Colloques, 1990, 51 (C6),

pp.C6-561-C6-564. �10.1051/jphyscol:1990673�. �jpa-00230943�

COLLOQUE DE PHYSIQUE

Colloque C6, suppl6ment au n022, Tome 51, 15 novembre 1990

W.S. LUCK, H.J. JANSCH,

D.

FICK and E. STEFFENS"

fhilipps-Universitdt, Fachbereich Physik, 0-3500 Marburg, F.R.G.

Max-Planck-Institut fiir Kernphysik, D-6900 Heidelberg, F.R.G.

T h e FILTEX-Experiment1 with the aim t o study the spin dependence of the p p interaction a t LEAR with a polarized p-beam and a polarized p t a r g e t requires a high density H-target. This can be achieved with a storage cell t e ~ h n i ~ u e ' ~ ~ . One of the basic problems of such a device is the depolarization of the stored R-atoms by a large number of wall collisions. In order to study this problem a polarimeter t o measure the nuclear polarization is essential.

P r i n c i p l e s

To start with we remark that the nuclear and the electronic polarization of hydrogen atoms in a weak magnetic field are identical, since each of the four hyperfine states contributes equally to the electron and the nuclear polarization (fig.1). Therefore a measurement of the electron polarization P, is sufficient to determine the nuclear polarization P,.

100 eV ^I

-

Pp=Pe=l

" ^{1 mT 6 field} 1 e l e c t r o n b e a m g

/

^{m W N N}

²

^l

Pp=Pe=-1

poLarized =-*

hydrogen

a t o m s circularly polarized fluorescence

B

^light

Fig. 1. T h e hyperfinestates of hydrogen atoms Fig. 2. T h e principle of the

i n a weak B-field. electron impact polarimeter.

To do so, the hydrogen atoms are excited by low energy electron impact (E,

=

80eV, fig. 2 ) . This excitation is so fast that the depolarization of the atomic electron by the excitation can be neclected. T h e state multipoles of the ensemble after the excitation ( T ( L , O)kLQr x T ( S ,

o ) $ ~ ~ ,

^X T ( I , O ) L I Q , ) separate t o ( T ( L , o ) ~ , ~ , ) ( T ( s , 0)Laqs ^X

T ( I , O ) L I Q , ) since the coherence between the orbital angular momentum and the spins is lost during the excitation.

T h e state multipoles of the orbital angular momentum (T(L, 0 ) L L q , ) depend on the excitation mechanism whereas the state multipoles of the electron and the nuclear spins ( T ( S , 0 ) L S Q , x T ( I , 0 ) k r ,) are not Jffected by the electron excitation and are equal to the multipoles before the excitation. T h e life time of %e 3P excited state is long enough for hyperfine coupling to transfer spin polarization to angular momentum polarization. T h e angular momentum state multipole evolves under the influence of the hypefine interaction Hamiltonian to

( T ( L ? ~ ) : , ~ , ) = 4 2 s

+

1)(2t

+

1)

C

( T ( L ? o ) ~ & ~ , X T ( S , 0)k5VS ^X T ( I ' o ) ~ ~ ~ ~ ) G ~ : ~ ~ I : : ~ ( ~ )

X s Q r , KsVs X 1 9 1

with the perturbation coefficients

~ V t V s V r n

K L K s K I L , ( t )

=

e ( i ( w 3 n - W ~ ' r ' ) ) - 1 / r ) t

-

^(2kl

+

^1)(2J1

+

^{1)(2 .l}

+

^1)(2F1

+

1)(2F

+

¹⁾

r

J I J X 3

- J ( ~ K L +

1)(2Ks

+

^1)(2K3

+

^1)(2Kr

+

¹⁾

-

( K L Q L , K s Q s ~ K J Q L -!- Q s ) ( K J Q L

+

Qs,KrQ~Ikiqr)

I F

a ) Supported by the Bundesminist-trium fur Forschung und Technologie, Bonn under contract 06MR1331

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990673

COLLOQUE

DE

PHYSIQUE

The circular polarization of the fluorescence light emitted a t the time t after the electron excitation is proportional t o the first rank polarization of the orbital angular momentum of the radiating atoms3

Using a close-coupling calculation of the electron excitation cross sections of the 3 P states4 we obtain the orbital angular momentum state multipoles a t the time t=O and calculate a circular polarization of the fluorescence light a t the time t shown in fig. 3. T h e calculation assumes a P, = P, = !jpolarization of the atoms a s i t is obtained with the one sextupole preparation used in our experiment. Fig. 3 shows oscillations of the polarization caused by quantum beats between the different magnetic sublevels of the n=3 level. Numerical averaging of the time dependent polarization shown in fig. 3 gives a light polarization of P=0.38

.

time after excitationhs

Fig.

3. Calculated light polarization a t t h e time t after the electron impact excitation.

A p p a r a t u s

Polarized hydrogen atoms are produced by a conventional source, using a SIN-type rf dissociator and one sextupole magnet5. T h e flux of hydrogen atoms into the interaction zone of the polarimeter is around 2.10'~ atoms/s. In order t o measure the background the hydrogen beam can be interrupted by a chopper wheel mounted close to the interaction zone of the polarimeter. T h e main source of background is the electron impact excitation of residual gas hydrogen molecules, leading t o dissociation of the molecules with one of the two hydrogen atoms ending u p in the n=3 state.

T h e

H2

residual gas is generated almost entirely by recombination of the atomic hydrogen beam on the surfaces of the vacuum chamber, of the storage cell and the chopper wheel (when the chopper is closed). T h e gas conductances inside the vacuum chamber are high enough that the background counting rate varies only slightly with the position of the chopper wheel. Thus a measurement of the background can be performed with closed chopper. T h e total error introduced to polarization measurements of storage cell targets by this method is estimated t o be less than 16%. This error can be reduced by increasing the pumping speed of the vacuum system.

A pair of Helmholtz type coils generates a magnetic guiding field in the interaction zone of ImT. T h e electron beam is produced by a simple and compact arrangement consisting of a n indirectly heated cathode (VALVO 30AX) and two electrodes. Electron currents in the mA range are obtained a t energies around 80eV. T h e induced fluorescence light is imaged by two lenses t o the cathode of a cooled single photon counting multiplier (RCA 31034). T h e H,-line is selected by an interference filter (Schott MA3-0.3). The circular polarization i s measured by means of a A/4 plate and a linear polarizer.

Our test set up to study wall depolarization with the new polarimeter is shown in fig. 4. For our polarization measurements we used a storage cell which could be removed during measurements on the free atomic beam for cali- bration purposes. To determine the polarization of the stored hydrogen atoms the polarization of the atoms effusing out of a n exit hole of the storage cell is measured. This way a meaningful determination of the target polarization is obtained since the atoms move randomly inside the storage cell. T h e storage cell measurements reported here were

erf formed

with a box shaped c d l providing a mean number of wall collisions around 1000. If the storage cell is cooled the target atoms move slower between the cell walls and the target density is increased. In order to investigate to which temperatures storage cells can be cooled without a loss of target polarization our test storage cell is connected to a closed cycle helium refrigerator. T h e accessible temperature range is 25K-300K.

M e a s u r e m e n t s

Fig. 5 shows the H, photon counting rate induced by electron impact excitation of the free atomic beam with a polarization of P,

=

P,

= $

a s a function of the position of the chopper wheel for right handed m+ and left handed

U- light. T h e counting rate is of the order of lkHz a t a n atomic beam intensity around 2.10'' atoms/s. This gives a sensitivity of the device of about 2 p h o t ~ n s / l O ' ~ ~ d r o ~ e n atoms. If the background is subtracted the measurement shown in fig. 5 gives a circular light polarization of P=0.172f 0.002 (statistical error) resulting in a n experimentally determined analysing power of the method around 0.34

.

This analyzing power turned out t o be independent on the energy of the electron beam in the energy range from 20eV to 80eV. Its dependence on the strengh of the magnetic guiding field is also weak.

We applied our method to measure the polarization of hydrogen atoms moving in storage cells. For a measurement on hydrogen atoms effusing from the storage cell the photon counting rate is of the order of 100Hz. Fig. 6 shows the polarization of the stored atoms normalized to the polarization of the free atomic beam t h a t was always measured seperately. T h e flux of the hydrogen atoms out of the cell that reaches the polarimeter normalized to the atomic beam flux is also displayed. T h e measurements show that fluorocarbon coatings like TEFLON, FLUOREL and

FOMBLIN

can be used for storage cell coatings a t temperatures down to 80K. At lower temperatures the target polarization and density decreases. Aluminum oxide coatings produce in all respects inferior results. T h e storage cells with fluorocarbon coatings showed target polarizations around 70% of the polarization of the free atomic beam. Remarkably enough no change of this value was observed when the storage cell was modified t o give 500 instead of 1000 wall collisions. The 70% polarization of the stored atoms compared to the free beam is presently not understood. Further investigations to clarify this point are presently under preparation.

D i s c u s s i o n

T h e measured light polarization when exciting the free atomic beam of P=0.172 is considerably smaller than the calculated value of P=0.38. This discrepancy probably re- sults from the fact t h a t the calculation does not indude ex- citation to higher states with subsequent deexcitation to the n=3 state. This deexcitation already produces polarized fluorescence light reducing the polarization of the subse- quently emitted

H,

photons. Another depolarization mech- anism t h a t is not included in the calculation is Stark mix- ing between the n=3 substates. The counting rate around

d

100Hz, the large analyzing power of 0.34, the weak depen- dency of this analyzing power on the electron beam energy and on the magnetic guiding field strength enables the a p plication of the method as a useful polarimeter for storage cells.

T h e ratio of t h e flux of hydrogen atoms effusing from the storage cell into the polarimeter to the atomic beam flux is about 1/10. This can be understood considering the angular distribution of atoms effusing out of a tube6. T h e drop of flux out of the storage cell a t low temperaturu can be described by a simple model assuming physisorption of the hydrogen atoms a t the storage cell surfaces with a n ac- tivation enthalpy of 1SmeV and a probability t o recombine to molecules proportional to the mean residence time of an atom a t the storage cell surface. T h e decrease of the polar- ization a t low temperaturu can be described similarly as

the shape of the intensity curve by assuming a depolariza- tion probability a t the storage cell surfaces proportional to the stay time a t the surface, although the absdute value of the polarization is not understood a t present.

T h e constant polarization of the stored hydrogen atoms in cells coated with fluorocarbons in the tempera- ture range 80K-300K allows to cool the storage ;ell down to 80K, resulting in a 90% increase of the target density as compared to room temperature operation.

Fig. 4. Apparatus used to study wall depolarisation in storage cells. T h e atomic beam enters the box-shaped storage cell a t the left side and is stopped

on a cone. T h e stored atoms partly leave the cell a t the right side and enter photo multiplier

the polarimeter.

COLLOQUE DE PHYSIQUE

Fig. 5. Photon counting rate induced by electron impact ^~mo excitation of the free atomic beam with a polarization of P,

=

P,

= $

as a function of the position of the chopper , rx,

wheel for right handed c+ and left handed U - light. ^+v-

.. ...

TEFLON

aoo

3

c. 0.05 ^-

a FLUOREL

cn c X

9

Q

0.05 - FOMBLIN

O.1°

1

^ALUMINIUM

1

1

FOMBLIN

storage cell temperature .T/K

Fig. 6. Measured flux (normalized to the flux of atoms into the cell) and polarization of hydrogen atoms effusing out of the storage cell.

r e f e r e n c e s

1 E. Steffens, Experiments with dense polarized internal targets, these proceedings 2 T. Wise and W. Haeberli, Workshop on Polarized Targets in Storage Rings 1984,

Argonne Nat.Lab.ANL-84-50, p.249

3 W. Jintschin e t al.,

l.

_Phys. B 17(1984)1899-1912 4 P.G. Burke et al., Phys. Rev. 129,3(1963)1258-1274 5 W. Korsch, diploma thesis, Universitzt Marburg 1987

6 J.A. Giordmsine and T.C. Wang, Journal of Applied Physics 31,3(1960)463-471