A MEASUREMENT OF THE 1s2p3PlLIFETIME IN HELIUM LIKE SILICON

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A MEASUREMENT OF THE 1s2p3PlLIFETIME IN HELIUM LIKE SILICON

I. Armour, S. Bashkin, N. Jelley, R. O’Brien, J. Silver, E. Trabert

To cite this version:

I. Armour, S. Bashkin, N. Jelley, R. O’Brien, J. Silver, et al.. A MEASUREMENT OF THE

1s2p3PlLIFETIME IN HELIUM LIKE SILICON. Journal de Physique Colloques, 1979, 40 (C1),

pp.C1-211-C1-214. �10.1051/jphyscol:1979143�. �jpa-00218423�

JOURNAL DE PHYSIQUE Colloque C1, suppl&ment au n o 2, Tome 40, fkvrier 1979, page C1-211

A MEASUREPENT OF THE 1 s 2 p 3 ~ 1 LIFETIME IN HELIUM LIKE SILICON

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I.A. Amour, S. ashk kin+, N.A. Jelley, R. O'Brien , J.D. Silver and E. Trabert University of Oxford, Clarendon Laboratory, Parks Road, Oxford

and

Nuclear Physics Laboratory, Keble Road, Oxford, England

:+ College of Liberal Arts, University of Arizona, Tucson, Arizona 85721, USA

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Melbourne State College, 757 Swanston St., Carlton, Victoria 3053, Australia

Experimentalphysik 111, Ruhr-Universitat, Postfach 102148, D-4630, Bochum 1 , F.R. Germany Abstract. We have observed the soft X-ray spectrum of highly ionised silicon and made measurements of the lifetime of the helium like system. Standard beam foil decay curve techniques were used and the

decay to the ground state was observed with a curved crystal X-ray spectrometer. A preliminary result of 6.68

0.40 ps was obtained which compares with theoretical estimates of 6.33 ps and 6.62 ps.

Rgsurne. Nous avons observe le s ectre des ions de silicium multi ionis;, et nous avons mesure la d u d e de vie du niveau ls2p3PI de Sil". On a utilia6 les techniques de beam foil spectroscopie, et la decroissance du

au niveau fondamental etait observ6 avec un spectrometre de Rayons X i cristal courbe. Notre resultat preliminaire est 6,68

0.40 ps, qu'on peut comparer avec les valeurs theoriques de 6,33 et 6,62 ps.

INTRODUCTION The principal allowed decay mode for the

In helium Like ions _tile I S Z ~ ~ P , state is mixed with the 1 s2plpl and other 'pl states by

fine structure interactions. The mixing is very small in helium but increases rapidly as we go to higher Z ions. A term scheme showing states of interest in the present experiment is shown in fig. I.

Fig.1 Term diagram of si12+

l s 2 p 3 ~ states is electric dipole to the 1 ~ 2 s ~ ~ ~ state (A

2 x lo8 s-l in silicon [ I ] ) . The

1s2p1P1 level however can decay directly to the ground state by electric dipole radiation, and has a very short lifetime (A

4

S-I in silicon [I]). Due to singlet triplet mixing, the

state can also decay directly to the ground state; the decay rate scaling approximately as 21°. The 3 ~ 2 , 0 levels do not mix with the singlet system and they have lifetimes lo3 times as long as

[I] , [2]. See also Drake [3] for details of magnetic quadrupole decay of the

state.

A study of the

decay rate provides a test of transition probabilities in this simple two electron ion.Therateis also of astrophysical interest, and such transitions may be used in measurements of electron densities in plasmas by

the

to

intensity ratio method of Gabriel and Jordan [4], [2]. The

lifetime has also

been studied by Varghese et a1 [5] using a Doppler tuned X ray spectrometer.

EWERIMENTAL ARRANGEMENT

The Si beam was supplied by the 6.7 MV EN Tandem Van de Graaff accelerator at the Oxford Nuclear Physics Laboratory. Beams of about 100 nA, at energies of 40 and 57 MeV, were

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979143

c1-212

DE

e x c i t e d by passage through carbon f o i l s , and decay c u r v e s o b t a i n e d by v a r y i n g t h e f o i l d i s t a n c e from t h e e n t r a n c e

of t h e s p e c t r o m e t e r . The experimental s e t up i s shown i n f i g u r e 2 .

Fig.2 Experimental arrangement

To r e s o l v e t h e

l i n e we used a 10 cm curved and b e n t c r y s t a l s p e c t r o m e t e r [ 6 ] ( s u p p l i e d

Applied Research Labs L t d ) . An

c r y s t a l of

0

l a t t i c e spacing 5.325 A was used t o a n a l y s e t h e

r a y s . The photons were t h e n d e t e c t e d i n a g a s flow p r o p o r t i o n a l c o u n t e r w i t h a I

polypropylene window u s i n g an argon methane gas mixture.

The s p e c t r o m e t e r was f i t t e d w i t h a d j u s t a b l e e n t r a n c e and e x i t s l i t s . By using narrow s l i t s we o b t a i n e d a r e s o l u t i o n of 0.003 fwhm g i v i n g a r e s o l v i n g power of 2000. Our l i f e t i m e measurements d i d n o t need such good s p e c t r a l r e s o l u t i o n , s o t o o b t a i n b e t t e r s t a t i s t i c s , wider e x i t s l i t s were used, g i v i n g a r e s o l u t i o n of 0.01 2 fwhm, a s shown i n f i g u r e 3.

5000 r

u

Fig.3 Spectrum a t 40 MeV w i t h .O1 A fwhm

The t a r g e t was a 10 pg cm-2 carbon f o i l which was moved i n 2.5

s t e p s by a s t e p p i n g motor.

The p o s i t i o n of t h e t a r g e t h o l d e r was measured t o w i t h i n

by a Heidenhain d i g i t a l l e n g t h gauge which u s e s a Mqire f r i n g e measuring g r a t i n g .

To observe t h e decay curve of a s h o r t l i v e d s t a t e we need good time r e s o l u t i o n . The time r e s o l u t i o n i s determined by t h e beam v e l o c i t y , t h e e n t r a n c e s l i t width, t h e angular acceptance of t h e s p e c t r o m e t e r and t h e d i s t a n c e of t h e beam from t h e e n t r a n c e s l i t s , a s shown i n f i g u r e 4. We used narrow e n t r a n c e s l i t s

um) and masked t h e c r y s t a l t o reduce t h e a n g u l a r acceptance of t h e s p e c t r o m e t e r . The i o n beam had i t s f i n a l c o l l i m a t i o n t o 1 mm x 2 mm on t h e t a r g e t i t s e l f and passed t h e e n t r a n c e s l i t of t h e spectrometer a t a d i s t a n c e of 1.5 ma. A decay curve showing t h e f a s t r i s e due t o good temporal r e s o l u t i o n i s shown i n f i g u r e 5.

0 bservat i on window

Fig.4 Shows dependence of time window on s l i t

width and e f f e c t i v e width of c r y s t a l .

To a l l o w f o r beam f l u c t u a t i o n s our s i g n a l

was normalised t o i n t e g r a t e d beam c u r r e n t . A

Faraday cup which moved a t a c o n s t a n t d i s t a n c e

of 3 ems from t h e f o i l was used, t h u s minimising

p o s s i b l e e r r o r s due t o change i n t h e geometry of

t h e charge c o l l e c t i o n system. The change i n

y i e l d of photons p e r u n i t charge due t o f o i l

a g e i n g was l e s s t h a n 1% over one l i f e t i m e of t h e

decay curve and thus had an insignificant effect on the lifetimes deduced from the measured curves.

An online data aquisition system controlled by a PDPlO computer via a CAMAC interface was used to control scans and take data. A typical scan would last about 4 hours collecting data at 5 or

10 urn intervals over a distance of 9 or 10

state lifetimes, i.e. just over 1 mm.

ANALYSIS

We analysed our decay curves by making a least squares fit of exponentials. Figure 5 shows a fit obtained by convolving a trapezoidal window function with the exponential decays, using a program developed by Tr'dbert and Winter [7].

S i

Fig.5

To remove the effects of small uncertainties in the window function, we cut off the first part of the decay curves. The loss in statistical precision introduced by this procedure was more than balanced by the elimination of systematic errors due to an imperfect description of the window function in the analysis programme.

To help show up possible systematic errors in our measurements, we worked at two different beam energies,.and with different masks

the crystal, to give different time windows. The results of fitting the decay with 2 cascades are shown in table 1.

Table 1: Measurements of the lifetime of the 2 3 ~ 1 state of Helium-like Silicon under different conditions

The short cascade was of negative amplitude and the longer cascade of positive amplitude. An unweighted mean gives a lifetime of 6.68 ps.

Fitting with only two exponentials (no short lived cascade) causes only a small increase in chi- squared but increases the scatter of results. It is interesting to note that when fitting only two exponentials the measured lifetime increases slightly with increased beam energy and might indicate systematic effects due to fitting too few cascades. This effect also seems to be present in the data of Varghese et a1 [5].

1

Observation window

short (~80 um) long (~110 um)

There are many cascades into the z3p1 state and our approximation of considering only 2 will introduce errors. We are making a more detailed analysis of our data in terms of expected cascades, which should result in a better

understanding of the sources of error. The present error quoted is much larger than the statistical error in the extraction of an exponential decay from one typical curve such as fig.5 and

corresponds to seven times the total spread in the results given in table 1. This error also

corresponds to twice the difference between the final result for the lifetime obtained using the three exponential fit, as in table 1, and the final result which is obtained if all the data are analysed using a two exponential fit.

RESULTS

Our results may be compared with the theoretical predictions of Johnson and Lin [8], Vainstein and Safranova [I] or with the previous experimental work of Varghese et a1 [5].

lifetime and cascades in ps (40 MeV)

6.70 1.0, 48

6.69 1.1, 57

lifetime and cascades in ps (57 MeV)

6.68 0.8, 30

6.64

4.2, 39

c1-214

Our present result is consistent with both theoretical values, although much closer to that of Vains tein and Safranova.

Experimental

Our 6.683.40 ps result

Varghese 6.35iO.33 ps et a1 [5]

REFERENCES 1. L.A. Vainstein and U.I. Safranova, Akademia 5.

Nauk CCCP,Inst.Lebedev, preprint no.6,

Moscow 1975. 6.

2. G.R. Blumenthal, G.W.F. Drake, W.H. Tucker, Astroph. J

1972) 205.

3. G.W.F. Drake,Astroph.J =(1969)1199. 7.

4. A.H. Gabriel and C. Jordan,Phys.Lett.32X(1970)

166 8.

A.H. Gabriel and C. Jordan,Case Studies in Atomic Collision Physics I1 (North Holland, Amsterdam( 1972) 21 1.

Theoretical Johnson

and Lin[8] 6.33 ps Vainstein

and 6.62 ps

Safranova [I 1

ACKNOWLEDGEMENTS

We would like to thank: Dr. M.A. Grace for his support and encouragement; Mr. J.C. Graham and the members of his workshop, particularly Mr. R.D. Wilson, for building the experimental chamber; and Mr. P. Wilcox Baker for building the length gauge interface.

S.L. Varghese, C.L. Cocke and B. Curnutte, Phys.Rev.fi(1976)1729.

I.A. Armour, S. Bashkin, N.A. Jelley, P. Kuske, R. O'Brien, J.D. Silver and E. Trkibert, submitted to J.Phys.B.(1978)

E. TrHbert, H. Winter, P.H. Heckmann and H.V. Buttlar, NIMS E(1976)353.

W.R. Johnson and C.D. Lin, Phys.Rev.2(1976)

565.