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ENERGY DEPENDENCE OF THE POLARISATION IN ZERO-FIELD QUANTUM BEATS IN Hβ
EMISSION
J. Carmeliet, J. Dehaes, W. Singer
To cite this version:
J. Carmeliet, J. Dehaes, W. Singer. ENERGY DEPENDENCE OF THE POLARISATION IN ZERO-
FIELD QUANTUM BEATS IN Hβ EMISSION. Journal de Physique Colloques, 1979, 40 (C1), pp.C1-
335-C1-337. �10.1051/jphyscol:1979171�. �jpa-00218451�
JOURNAL DE PHYSIQUE Colloque C1, supplkment au n O 2, Tome 40, fkvrier 1979, page Cl-335
l3EEw;u EPEhBENE OF THE POIARISATION IN ZUM-FIELD QUAN?JM BEATS IN Hg EMISSION J. camelietM, J.C. Dehaes and W. Singerm
Universit6 Libre de Bruxelles, Service de %trologie Nuclsaire, Avenue. F .D
.
Roosevelt,
50, 1050 Brussels-
Belgium~ s
-
k s battements quantiques d 3 champ nu1 ont 6t6 observi5s sur l a raie H (4 861 A) de l'hydroghe -par I t i n t e r a c t i o n faisceau-lame de protons de 110 2 250 kev. Nws en atom &duit les sections ef-ficaces d' excitation relatives dans l e s sous-niveaux magn6tiques des e'tats 4s, 4p e t 4d en fonction de 1' 6nergie. La polarisation&-Il moyenn6e sur les oscillations e s t n 6 p t i v e . A l'accasim de ces msu- res, nous avons 0bservi5 I' uence de l'enviromement i d d i a t de l a f e u i l l e de carbone. Cela s ' e s t ma- nifesti5 par une polarisation moyenne powant devenir positive e t par des oscillations d'amplitude plus importante. Nous powons attribuer ce phe'nomsne aux potentiels e'lectriqws dus aux courmts c i r c u l m t dans l a f e u i l l e e t son support. Nous avons mis en Bvidence l'importance de l a nature de l a surface du support e t de l a r 6 s i s t i v i t 6 de l a f e u i l l e .
Abstract
-
Zero-field quantum beats were observed i n t h e 3 emission (4 861 11) from hydrogen atoms for- m e a m - f o i l interaction of 110 t o 250 kev proton be.
The energy dependence of the relative exci- tation cross sections for the magnetic substates of the 4s, 4p and 4d levels was derived. The polarisa- tion 11-II averaged over the oscillations is negative. Along with these measurements, we observed the influence of the immediate neighbourhood of the carbon f o i l . This was seen by increased oscillation am- plitudes and an averaged polarisation which might becom positive. We a t t r i b u t e t h i s phenomnon t o elec-t r i c a l potentials due t o currents circulating i n the f o i l and i t s holder. The importance of the surface nature of the holder and the r e s i s t i v i t y of the f o i l was pointed out.
INTRODUCTION
The energy dependence of the ~ o l a r i s a t i o n in zero-field quantum beats i n Hp emission
(4 861 A) has been studied. The hydrogen atoms are f o m d by beam-foil interaction of 110 t o 250 kev pro tons.
The H+ ion beam is produced by a 300 kV S m s ac- celerator. The carbon f o i l (10 vg/un 2 ) is moved by a stepping motor with steps of 0.5 nnn on a
length of 80 mm.
The l i g h t emitted perpendicularly t o the beam i s detected through an i n t e r f e r e n t i a l f i l t e r (Fil- traflex-B 40,h = 4 870
a,
= 80a)
by a pho- tomultiplier (EMI 9502 S) i n front of which we have put a l i g h t pipe, because of the inhomogenei- t i e s of the photocathode.The image (1x1) of a beam portion of 0.5 nnn large is fogned on a v e r t i c a l s l i t used as diaphragm.
The image of the s l i t is focused (1x1) a t the entrance of the l i g h t pipe.
A linear polarise? -(HN42) may be put p a r a l l e l or perpendicular to the beam by a stepping motor.
The protons are collected by a Faraday cup prece-
grounded t o earth. The magnetic f i e l d i n the t a r - get chamber is reduced t o about 0.02 G i n both transverse directions t o the beam, by a s e t of two couples of coils.
For each position of the f o i l we measured thelight signal (Il ,Il) polarized perpendicularly and v e r t i - c a l l y t o the beam
.
These signals are amplified and stored alternatively i n two separate Emory- blocks of a multichannel analyser (SEIN-
ZOOMAX).The integration t i m e i s determined by the proton current.
EXPERIMEWAL RESULTS
From the measured signals I/ and Il
,
we ha- ve calculated the polarisation P = Il-IL and the t o t a l intensity I t = 11+211.
P and It, f o r Hp,
are given by the expressions :
ded by two shields : one a t -100 V and the second I t = Bs eq(-rst)+B exp(-r t)+Bd e q ( - r d t )
P P
R Work supported by I n s t i t u t Interuniversitai-re
des Sciences Nuclgaires
rR
is the decay constant of the 1 s t a t e and hwChercheur technique de 1'I.I.S.N. ~j
Chercheur qualifi6 de 1'1 .I .S .N. the energy of the a j s t a t e .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979171
C1-336 JOURNAL DE PHYSIQUE
Fig. 1
-
Results of l e a s t square analysis of the polarisation (Il-
11) i n function of the proton energy.(a), (b), (c) amplitudes A Ad and Asd normalized to a t o t a l intensity I t a t the f o i l of 100 photons. (d) the phase of P' the s-d beats assuming zero phases f o r p and d beats.
t On both figures, (x)
,
(+),
(.) represent threeseries of measuremnts made a t different tilnes
-
f ' +
and (0) i s the result of the Lyon experiment[ 41.Fig. 2
-
Calculated average polarisation i n function of proton energy.and Ad are proportional t o the alignmnt of the p and d s t a t e s . Asd is the sd coherence term.
B, is proportional t o the i n i t i a l population of the a s t a t e .
For each measurement, we made a Fourier transform of P. The only frequencies observed were those which appear in the expression of P (7371 .I
,
457.03, 1236.3 and 1693.3 MHz)
.
We also made the l e a s t square analysis of the po- larisation using the P expression as a f i t t i n g function with s i x parameters. Four of them are the amplitudes
+,,
Ad, Asd ant the phase 4sd. The two others, the energy of the beam and the position of the f o i l , were added because they are known with same uncertainty. The frequencies u and decaya j
constants
ra
were taken from H.A. BE'IHE and E.E.SALPETER [ I ]
.
The results are shown on the figure 1 i n the energy range 110-250 kev. We have also calculated the a- verage polarisation
P
= 0.5 A + 19/6 Ad. Figure 2 shows thatP
remains negative. PThe alignmnts of the p and d s t a t e s
(%,
Ad) de- crease with energy. A is always negative. Ad i s quite small and seem t o be zero a t about I50 kev. P A similar decrease of%
has been measured f o r the Lya previously [ 21,
[ 31.
The results of A. LENIS e t a l . [ 41 f o r 250 kev are reported on Fig. 1 and 2. The agreement with our r e s u l t i s not very good. Their A for instance is half ours. P
Nevertheless the average polarisation and the phase $sd f i t in very well.
TNFLUENCE OF STRAY ELE(;TRIC FIELDS
We have observed that the presence i n the v i d - nity of the carbon f o i l of any material has an im- portant i n f l ~ n c e on the meakurements. The average polarisation might become positive and the oscilla- t i o n amplitudes increase. The f o i l was mounted on a holder (5x15 mm 2 ) and placed on a brass sheet with a 4 nun hole. Two kinds of experiments have been done. In the f i r s t case, we varied the rela- t i v e position of the f o i l holder and the sheet.
P
i s negative when the f o i l hol&r i s a t about 10 nun from the sheet and becomes positive a t about 3 nun. In the second case, the f o i l holder was be- hind the sheet and we changed the nature of the surface of the sheet forwards. The sheet was cove- red by CuO, black paper and carbon, There is a marked oscillation on the t o t a l intensity It.These f a c t s may be attributed t o an e l e c t r i c a l f i e l d due t o the potential drop between the f o i l and the sheet. The potential of the f o i l , a t the beam im- pact, and a t the holder depends on the beam cur- rent (5-10 d)
,
on the nesistivity of the carbon (one t o several hundreds of Qcm) and on the con- t a c t resistance a t the holder. The potential of the sheet depends on the e l e c t r i c a l properties of the surface covering and the electron current im- pinging on the sheet. The potential drop may be- come quite high (several V/cm) which is sufficient in the case ofHp
t o mix up levels with different a by stark effect.BIBLIOGRAPHIE
1 11 HiA. Bethe, E .E. Salpeter, Quantum Mei-hanics [31 H. Winter, H.H. Bukow, 2 . Physik A 277, p.27 of one- and two-electron at-, Springer- (1 976)
.
Verlag Berlin, p. 266 (1975)
.
141 A. Denis, J. Desesquelles, M. Druetta, M.[ 21 P. Dobberstein, H. J. AndrP, W. Wittmann, 2 . Dufay, Beam-Foil Spectroscopy 2, 6d. I .A.
Physik
