A MEASUREMENT OF THE 1s2p3PlLIFETIME IN HELIUM LIKE SILICON

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 3 ~ 1 decay to the ground state was observed with a curved crystal X-ray spectrometer. A preliminary result of 6.68 f 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 dude de vie du niveau ls2p3PI de Sil". On a utilia6 les techniques de beam foil spectroscopie, et la decroissance du 3 ~ 1 au niveau fondamental etait observ6 avec un spectrometre de Rayons X i cristal courbe. Notre resultat preliminaire est 6,68 2 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 2p3~ states is electric dipole to the 1 ~ 2 s ~ ~ ~ state (A 5 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 x S-I in silicon [I]). Due to singlet triplet mixing, the 3 ~ 1 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 3 ~ 1 [I] , [2]. See also Drake [3] for details of magnetic quadrupole decay of the 3 ~ 2 state. A study of the 3 ~ 1 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 3~ to 3~ intensity ratio method of Gabriel and Jordan [4], [2]. The 3 ~ 1 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 JOURNAL DE PHYSIQUE exci ted by passage through carbon f o i l s , and decay curves obtained by varying the f o i l d i s tance from the entrance s l i t of t h e spectrometer. The experimental s e t up i s shown i n f i g u r e 2. Fig.2 Experimental arrangement To reso lve the 3 ~ 1 l i n e we used a 10 cm curved and bent c r y s t a l spectrometer [6](supplied by Applied Research Labs Ltd). An ADP c r y s t a l of 0 l a t t i c e spacing 5.325 A was used t o analyse the X rays. The photons were then detected i n a gas flow proport ional counter with a I pm polypropylene window using an argon methane gas mixture. The spectrometer was f i t t e d with ad jus tab le entrance and e x i t slits. By using narrow s l i t s we obtained a reso lu t ion of 0.003 fwhm giving a resolving power of 2000. Our l i f e t i m e measurements did no t need such good s p e c t r a l reso lu t ion , so t o ob ta in 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, giving 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 pm s t e p s by a stepping motor. The p o s i t i o n of the t a r g e t holder was measured t o within ?1 pm by a Heidenhain d i g i t a l length gauge which uses a Mqire f r i n g e measuring grat ing. To observe the decay curve of a shor t l ived s t a t e we need good time reso lu t ion . The time reso lu t ion i s determined by the beam ve loc i ty , t h e entrance s l i t width, t h e angular acceptance of t h e spectrometer and t h e d i s tance of t h e beam from t h e entrance sl i ts , a s shown i n f i g u r e 4. We used narrow entrance s l i t s (-70 um) and masked the c r y s t a l t o reduce t h e angular acceptance of the spectrometer. The ion beam had i t s f i n a l col l imation t o 1 mm x 2 mm on the t a r g e t i t s e l f and passed the entrance s l i t of t h e spectrometer a t a d i s tance of 1.5 ma. A decay curve showing the f a s t r i s e due t o good temporal reso lu t ion 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 slit width and e f f e c t i v e width of c r y s t a l . To allow 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 in tegra ted beam cur ren t . A Faraday cup which moved a t a constant d i s tance of 3 ems from t h e f o i l was used, thus minimising possible e r r o r s due t o change i n the 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 per u n i t charge due t o f o i l ageing was l e s s than 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 3 ~ 1 state lifetimes, i.e. just over 1 mm.