nucleon energy: Preliminary results

NUCLEAR INSTRUMENTS

AND METHODS 9°

(I97O) 85-9I;

© NORTH-HOLLAND

PUBLISHING

CO.

BEAM-FOIL S P E C T R O S C O P Y AT 1 MeV/NUCLEON E N E R G Y : PRELIMINARY RESULTS M. D U F A Y , A. DENIS and J. DESESQUELLES* Laboratoire de Physique de l'Atmosphdre, Equipe de Recherche associde au C.N.R.S., Facultd des Sciences de Lyon, 69 - Villeurbanne, France We report the preliminary results of our measurements on the spectra obtained by very high energy beam-foil interaction, Beams of carbon, oxygen, nitrogen, neon and argon ions were supplied by the Orsay linear accelerator.

Mean lives of several excited levels in hydrogen-helium-and lithium-like ions of the aforementioned elements were measured.

1. Introduction One of the really promising developments of beamfoil spectroscopy is concerned with the study of highly ionized heavy ions of astrophysical interest. With present day source technology, the use of Van de Graaff machines is difficult in that field. At lower energies the problem of the ion source has been solved successfully but the stripping by carbon foil becomes inappropriate for the study of highly ionized ions. Fortunately, the foil excitation technique can be used efficiently on heavy ion beams produced by linear accelerators. In that case, the sources developed for high energy nuclear physics are readily capable of producing all the ions of astrophysical interest (mostly metallic ions). The particularity of the accelerating process (fixed q/m, where q is the charge and m the mass of the ion) insure a high purity of beam and a fixed final speed independent of the mass of the accelerated products. Berkner et al. l) were the first to investigate beam-foil spectroscopy in such new experimental conditions. Their published results, however, are concerned only with members of the lithium isoelectronic sequence of low ionization stage which could have been obtained just as easily with a 6 MeV Van de Graaff. We have recently been able to use for a short period of time the 1.15 MeV/nucleon Orsay linac. Low resolution spectra of multionized C, N, O, Ne and Ar were recorded. Attempts were made to measure the mean life of several levels in the observed hydrogen-like, helium-like and lithium-like ions.

It will soon be supplemented with an oven enabling us to introduce metallic vapors of all nature in the source. For the time being, the intensity of the beam is of the order of several/zA. Due to conflicting requirements imposed on this multipurpose equipment, we carried out our experiment on a poorly focused part of the beam. In order to increase the light output we used broad self supporting carbon foils (20 mm in diameter) with densities ranging from 25 to 150/~g/cm 2. In the visible (for Ne and N on/y) our standard observation apparatus includes quartz window, quartz lens and Hilger 0.60 m CzernyTurner spectrometer with EMI 6256 S photomultiplier. In the ultraviolet with a LiF window, we used the 0.30 McPherson vacuum spectrometer with an Ascop E M R 541 F photomultiplier. In both cases, we used direct current amplification and strip chart recordings. For lifetime measurement purposes the foil could be moved along the axis of the beam over 20 cm.

2. Experimental apparatus The "Alice" Orsay linear accelerator is the injector of the "Cevil" machine 2) (variable energy heavy ion cyclotron). It is of the Sloan-Lawrence type and is able to accelerate ions with q/m >=O.l, for example C 2+, N z+, 0 z+, Ne 2+ and Ar 4+. The multi-ionized ion source is of the Morozov type and can presently deliver beams of ions with mass as high as krypton.

3. Spectra The beam-foil source was extremely weak in our experimental conditions. With two centimeters slit height, we have never been able to reduce the slit width under 1 mm. This leads to a poor resolution (20 ~/mm) which is, however, quite appropriate if one takes into account the large Doppler broadening of the observed lines. Indeed, at 1.15 MeV/nucleon, f l = v/c = 0.05 and the Doppler width of a line at 3000 is 25 ~. with our observation settings (fig. 1). Our analysis of the observed spectrum was simplified by the previous measurements of Baron 3) on the charge state composition of the beam after the foil (see table 1). It was then clear that we should observe hydrogen-like, helium-like and lithium-like lines for the light elements (C, N, O and Ne). The Argon case must be considered apart. 3.1. ONE-ELECIRON IONS ( C VI, N VII, O VIII, Ne X) As a first step in the analysis of the recordings we used the tables of Garcia 4) and were consequently able

* Presented the paper.

85 II. LIFETIMES AND T R A N S I T I O N P R O B A B I L I T I E S

~6

M. DUFAY et al. 1000

1500

1125

N Vl

4-5

1240

NV

2s-2~

1530

NVII

5-6 6_8

1620

NV

4-5

1900

NVI

2s-zp

N Vl

5-6

NVII

7-9

NVII

6_7

NV

5_6

2070 2310

2520

2500,

2980 3000.

-

3140

3440

NVI

B-7

3500,

3880

NVII

7_8

4000'

Fig. 1. Nitrogen spectrum at 16 MeV energy. not come as a surprise since it is well k n o w n that the collisional excitation o f the singlet is more difficult than the excitation o f the triplet system. I n order to go further in the identification process, we c o m p a r e d the spectra obselved in the same conditions with carbon, nitrogen, oxygen and neon. The wavelengths o f several lines obtained with comparable intensities in the four spectra exhibited a 1/Z 2 dependence with Z = 5, 6, 7 and 9 respectively. This was clearly a p r o o f o f their belonging to the two-electron spectra o f the considered elements, more precisely,

to identify several transitions o f the C V[, N VII and O V I I I spectra. The corresponding identifications in Ne X could not be made unambiguously. In the observed spectral range, the recorded transitions correspond to high lying levels n __>6. They are listed in table 2. 3.2. TWO-ELECTRON IONS (C V, N VI, O VII, Ne IX) The only tabulated lines o f these spectra (fig. 2) in our spectral range were the 2s3S-2p~P ° and 2 s I S - 2 p t P ° transitions. We observe only the triplet and this does

TABLE 1 Charge state composition of the beam stripped by the foil, in per cent. Ion N O Ne Ar

X ~5.7 6.18 7.4 11.6

4+ 4

5+ 37 14

6+

7+

8+

46 56.2

13 26.2

3.3

9+

10-1-

11+

12+

13+

14+

2.6

12.8

29.7

32.7

17.9

4.1

B E A M - F O I L S P E C T R O S C O P Y .AT 1 M e V / N U C L E O N

87

ENERGY

TABLE 2 Spectra and mean lives in C, N, O, Ne and Ar ions. Element

C

N

Measured wavelength (•) 1480 1540 1625 2015 2075 2275

Predicted wavelength (A)

Transitions

w m s

1548

2s'2S-2p2p ° 4f3F°-5gZG

IV V

w s

2071 22?4

5-6 2saS-2paP °

VI V

1239

4f3F0-5gaG 2s~S-2peP °

VI V

1125 vw 1240 s 1530 m

O

Ne

Ar

1620 1900 2070 2320 2430 2520 2980 3140 3440 3880

w m m w w s m m vs m

1170 1210 1370 1520 1630 1910 2070 2310 2530

w m w s vs w s w s

1255 1390 1530 1780 1950 2260 2360 2540 2770 2980 3435 3645 4030 4340

s w m vw m m s s w m m m m s

1220 1330 1540 1640 1920 2310 2690

m m m m m s m

ionization state

5-6 1620 (1901)

2981

1216

(1630)

(1297)

(1930)

1216

VII

4fd-5gf 2szS-2pzP ° 5-6 7-9

V VI VI VII

Meanlife (ns) This work Other a u t h o r s

0.378 •

11.4 3.4 ± 0.2 0.3 (1.8) 0.68 (3,8) 13.8 0.5 (1.9) (7.5)

6-7 5-6

VII V

0.8 (6.8)

6-7 7-8

VI VII

1.23 (3.4) 1.5 (19)

5-6 L y m a n cz 2s2plP°-2p 2 ±D 5-6 2sSS-2p3P ° 6-8 5-6

VIII V VII VII VIII VI

6-7

VII

2s3S-2p3P °

IX

6-7

IX

6-7

VIII

7-8

0.37 (1.5) (15) 15.1

17.5 b / 2.6 a 2.97 u / 3.17 e 0.22 a (6s) 0.25 a (6h) 0.38 a 14.7 b 0.476 ~ (6h)

1.06~ (7i)

0.257 ~ 12.6 t'

0.6l (3.8)

12.6

10.2 b

0.2 (0,9) (9.7)

0.21 a

iX

0.55 (6.1)

0.419 a

7-8

VIII

0.94

8-9

VIII

Lyman

Bates and D a m g a a r d calculation11). b Weiss calculation12). c Bickel measuremenOO). The wavelengths values in brackets are extrapolated. The meanlives values in brackets represent cascade. vs - very strong; s - strong; m - m e d i u m ; w - weak; vw - very weak. II. L I F E T I M E S A N D T R A N S I T I O N

PROBABILITIES

88

M. DUFAY et al. 3S

3p

3D

3F

G

H

I

K CV

NVI

OVIINelX

2360

8k 7i

4.950J34 40 2 5 3 0 1 5 3 5

~_~ 59

.............. 2 0 7 0 2960

520

162C~11 2 5

4f

2275190£ 2s

He-like

6 3 0 !26(3

ions

Fig. 2. Helium-like ions diagram. The lines already observed at 2 MeV energy are underlined. they must be attributed to transitions with An ¢ 0 of the corresponding helium-like ions. Extrapolation of levels 5) along the isoelectronic sequence indicates that the observed lines are due to the desexcitation of high lying levels (n = 5 to 8) and indeed, we knew from our previous study of Li 1I 6) at Van de Graaff energies that the beam-foil excitation mechanism was able to populate heavily such levels. 2S

2p

2D

2F

G

It is difficult to make more precise identifications since the splittings of the sublevels are very small for high principal quantum numbel levels. F r o m branching ratio arguments, however, it is possible to conclude that the most intense lines in our spectral range should correspond to the 4f-5g, 5g-6h, 6h-7i and 7i-8k transitions. Consequently, the intense line of the nitrogen spectrum appearing at 3430 • is attributed to the H

I

K CIV

NV

OVl NeVIII 2980 1950

2980! 2070

5g ;16"20

4f

? 2s

2p

1548 1240

Li - l i k e

ions

Fig. 3. Lithium-like ions diagram.

BEAM-FOIL SPECTROSCOPY

AT 1

MeV/NUCLEONE N E R G Y

200

89

~ 2 p 124oi

25 ..J o tN tN

~,~ I00 Z LIJ I.--z

to o

50

LLI i-< -J LL/

\

20

10 I i

31000

0

lo'oo

2000

5

10

DISTANCE

FROM

15

FOI

L

(cm)

Wavelenglhs

t=ig. 5. Decay of 2p2P ° level in N V at 1240 ]k (16.6 MeY). Fig. 4. A r g o n spectrum at 46 MeV.

>-

NVI

20 u3 z LIJ i--z

2s_2p

1900

,~

10 --....,..

LLI m


5

LIJ

2 0

,

l

±

I

5

10

15

20

DISTANCE

FROM

FOl

L

(cm)

Fig. 6. Decay of 2p~P ° level in N VI at 1900 ,~ (16.6 MeV.) II. L I F E T I M E S A N D T R A N S I T I O N

PROBABILITIES

90

M. DUFAY et al. above are applicable here. The energy gap between our previous studies at 2 MeV maximum and the present measurements at 40 MeV is such that all the recorded lines (see fig. 4) are in fact observed for the first time. The only indications that we could rely upon concern their charge state which, from the results of Baron3), is between Ar X and Ar XV.

100 NVI

e-7

3440

,~

50

4. Mean life measurements >l-u') Z LLI I.--

20

Z

W > I0 I.-< .-I Ld w. 5

\

I

0

5

DISTANCE

10

FROM

15

FOIL

(cm)

Fig. 7. Decay of n= 7 level in N VI at 344 ~ (16.6 MeV). 6h-7i transition in N VI. This result is indirectly confirmed by our previous observation of the corresponding C V line at 4950 ~ by beam-foil spectroscopy of carbon at lower energies. Other identifications of helium-like transitions are listed in table 2. 3.3. THREE-ELECTRONIONS (C IV, N V, O VI, Ne VIII) The identification of lines belonging to these ions (fig. 3) was greatly simplified by a direct comparison of our recordings with the results 7-~) obtained previously in our laboratory with a 2 MeV Van de Graaff accelerator. There is a complete continuity between the sets of results. The corresponding identifications are listed in table 2. One should remark particularly that the lines of N V are still quite intense at 15 MeV.

With our setting, the motion of the foil along the beam axis was restricted to 20 cm which is a short distance if one considers the high velocity of the emitting particles. Furthermore, the low duty cycle of the linear accelerator forbid the use of large integration times. It has not been possible to gather any meaningful statistics and consequently the lesults listed in table 2 are only very preliminary estimates. Keeping in mind these important experimental limitations, it is interesting to note that our result on the 2p2P ° levels of N V (fig. 5) agrees closely with the previous measurements of Bickel et al. 1°) at 2 MeV and Berkner et al. 1) at 15 MeV. Among the helium-like levels, the 2s3S-2paP ° transitions seem to be cascade free, but the lifetimes of the 23P ° levels are not determined with good precision because they are in average too long for the available length of displacement of the foil (figs. 6,8). Cascade effects were apparent on the other measured heliumlike transitions (fig. 7). Whenever possible, we tried to interprete these cascades using the diagram of the upper levels (fig. 2). For example in N VI the recorded decay curve of the level 7i (3430 A) has been success-

2 s~S - 2 p3pO Hel

o12 i

isoeleefronic

• This work * M.C.Poulizac • I. M a r t i n s o n A , W . W e i ss

sequence

ef al.

0.1,

l/z NelX

3.4. ARGON CASE None of the identification techniques mentioned

OVH

NVl

CV

Fig. 8. 2s3S-2pzP° oscillator strengths in the He I isoelectronic sequence7,12,lz).

B E A M - F O I L S P E C T R O S C O P Y AT 1 M e N / N U C L E O N

fully used for the interpretation of the 6h level decay curve (2070 A). The case of the hydrogen-like transitions corresponding to high principal quantum number upper levels is much more complicated. The degenerate sublevels have widely different meanlives. The values indicated in the table correspond in fact to the longer components (s-sublevels), the decay of the other sublevels being too fast to be recorded. Also indicated in the table are the results of the computations by Ceyz6riat 1~) in the Bates and Damgaard approximation which is known to give good results for highly ionized ions. The agreement is satisfactory for most of the studied transitions. It seems, however, that the long decay constant measured on the 6-7 transition of O VII corresponds in fact to the 7-8 cascade transition which has a predicted lifetime of about 1.5 ns whereas the theoretical value of the 6-7 transition meanlife is only 0.57 ns. 5. Conclusion With these preliminary results on the beam-foil spectroscopy at 1 MeV/nucleon we achieve two main objectives: 1. To verify that without basic change in our technology it will soon become possible to measure with good reproducibility mean lives of excited states in very energetic ion beams. 2. To confirm that the energy increase obtained by use of a linear accelerator instead of the classical Van de Graaff machine is sufficient to enable us to reach by beam-foil interaction highly stripped ions of astrophysical interest. The main difficulty now comes from the non-adjustable character of the energy of the beam. Identification of charge states from the line intensity variation with energy is then impossible. Best results are obtained from that point of view when a continuity solution can be found between lower energy observation and the high-energy data. The authors wish to express their gratitude to Professors J. Teillac and M. Lefort for the opportunity of working on the Orsay Linac machine. They also wish to extend their appreciation to Chief Engineer Dr. A. Cabrespine for his highly qualified collaboration.

ENERGY

91

References 1) K. Berkner, W. S. Cooper IIl, S. N. Kaplan and R. V. Pyle, Phys. Letters 16 (1965) 35. 2) A. Cabrespine, J. Phys. Suppl. 5-6, no. 30 (1969) C 2. 3) E. Baron, private communication. 4) j. D. Garcia and J. E. Mack, J. Opt. Soc. Am. 55 (1964) 654. 5) C. Moore, Atomic energy levels, Report Nat. Bur. Std. 467 (1949). 6) F. Galliard, Thesis (Lyon, 1969). 7) M. C. Poulizac and M. Druetta, Compt. Rend. Acad. Sci. Paris 270 (1970) 788, and private communication. s) j. Desesquelles, Thesis (Lyon, 1970). 9) M. Druetta, private communication. 10) W. S. Bickel, H. G. Berry, J. Desesqueiles and S. Bashkin, J. Quant. Spectr. Radiative Transfer 9 (1969) 1145. 11) p. Ceyzeriat, private communication. 1-2) A. W. Weiss, in Atomic transition probabilities, Nat. Bur. Std. 4 (1966). 18) I. Martinson and W. S. Bickel, Phys. Letters 31A (1970) 25.

Discussion BERGSTROM" ~-[OWdo you measure the particle velocity? DESESQUELLES: All ions from the Orsay linear accelerator have the same velocity, about 1.5 cm/ns. HALLIN: Why did you use such thick foils? DES~SQUELLES: Foils of thickness up to 100 gg/em 2 are needed to produce charge equilibrium at such high energies. QUESTION: HOW did you identify the charge states, responsible for the various spectral lines? Also, what was a typical time for accumulating one spectrum? DESESQUELLES: Very often isoelectronic comparisons were used. A typical recording time was 15 min. SELLIN: Which part of the beam was viewed by the spectrometer? DESESQUELLES: Two positions were used for viewing in the side-on configuration: one approximately 1 m m from the foil and the other approximately 6 m m from the foil. Some spectral lines increased in intensity between the first and the second position before decaying in intensity further away from the foil. GABm~L: I am worried by the references made at this meeting to the fact that many of the transition probabilities measured are of astrophysical interest. There are two problems here. Firstly, for the neutral and singly ionized lines occurring in photospheric absorption spectra, transition probabilities are indeed important, and we are all aware of their vital contribution recently to the question of iron abundance. For highly-charged ions the situation is different. For these, in the non-L.T.E, conditions occurring in astrophysics, spectral intensities are determined by eollisional excitation rates and not transition probabilities, f-numbers are used in some approximate theoretical expressions for excitation rates, but for these, Coulomb-approximation f-numbers are certainly adequate. Intensities do depend on transition probabilities for long-lived metastable levels, however, where in any case the Coulomb approximation method is not available. I would therefore make a plea for the measurement of transition probabilities for intersystem and forbidden transitions in these highly charged ions.

II. L I F E T I M E S AND T R A N S I T I O N P R O B A B I L I T I E S