AXIAL OBSERVATION OF X-RAY SPECTRA FROM A BEAM-FOIL SOURCE

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AXIAL OBSERVATION OF X-RAY SPECTRA FROM A BEAM-FOIL SOURCE

J. Laming, J. Silver, R. Barnsley, J. Dunn, K. Evans, N. Peacock

To cite this version:

J. Laming, J. Silver, R. Barnsley, J. Dunn, K. Evans, et al.. AXIAL OBSERVATION OF X-RAY

SPECTRA FROM A BEAM-FOIL SOURCE. Journal de Physique Colloques, 1988, 49 (C1), pp.C1-

339-C1-342. �10.1051/jphyscol:1988173�. �jpa-00227587�

AXIAL OBSERVATION OF X-RAY SPECTRA FROM A BEAM-FOIL SOURCE

J.M. LAMING, J.D. SILVER, R. BARNSLEY' , ^{J. DUNN*} , K.D. EVANS' and N.J. PEACOCK"

University of Oxford, Clarendon Laboratory, Parks Road, GB-Oxford OX1 3PU, Great-Britain

'University of Leicester, Department of Physics, University Road, GB-Leicester LEI 7RH, Great-Britain

' * ~ u l h a m Laboratory, GB-Abingdon OX14 3DB, Oxon, Great-Britain

Abstract: New observations of x-ray spectra from foil-excited heavy ion beams are reported. By observing the target in a direction along the beam axis, a n improvement in spectral resolution, 6A/A, by about a factor of two is achieved, due t o t h e reduced Doppler broadening in this geometry.

As pointed out some time ago('), by observing t h e target in a direction along the ion beam axis, the Doppler broadening of spectral lines is essentially due t o the spread in ion velocities in the beam, rather than due t o the ion beam divergence as would be the case for viewing perpendicularly t o the ion beam direction. T h e reason for this can be seen from a consideration of the following set-up shown in figure 1. If a n ion moving with velocity pc, where c is t h e speed of light, a t a n angle a t o the beam axis emits radiation with wavelength Xo, the Doppler shifted wavelengths seen by the two observers are;

A' = 7Xo(l

+ p

sin a ) (perpendicular) (1) A' = yXo(l -

p

cos a) (axial) (2) where 7 = (1 -

p2)-lt2.

If we put the actual spread in wavelengths seen;

then taking 6a

-

^{5 x}

1 0 - ~ , p -

^0.06,6/3

- ^low4,

(typical conditions on the Oxford Folded Tandem accelerator), gives;

6A/Xo

-

^{3 x} (perpendicular) (4)

6A/Ao

-

^{1 x (axial) ( 5 )}

giving a n improvement in resolution of approximately a factor of three. If 6 P / p can be reduced one might expect an even greater improvement. This axial viewing technique is demonstrated in Si12+, where the He-like resonance and intercombination lines are observed along with a number of satellite transitions, and in t h e best resolved NeQf Lyman a doublet t o date. T h e spectrometer used in both of these observations was a curved crystal instrument with a variable Rowland circle radius, generally used between 1 and 2 m, which was developed by the Radiation Physics Group a t the University of Leicester. T h e flat crystals of aperture 20 mm by 25 m m were bent in an original four-bar beam-bending jig. T h e generated optical focus was greater than A/6X

-

30000. The Rowland width

and height aberrations were X/6X

-

30000 and X/6X

-

14000 respectively. T h e dominant instrumental broadening was therefore the rocking curve of the crystal used. T h e spectra were recorded on double-sided Kodak DEF 392 x-ray film.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988173

JOURNAL DE PHYSIQUE

Ion trajectory Target

Axial observer

v

Perpendicular observer

Figure 1. Schematic diagram of beam-foil geometry.

The source of ions for the experiment was the Oxford University Folded Tandem accelerator. A beam of Sisf ions at 63 MeV was stripped and excited in a 10 Icg/cm2 carbon foil. Radiation from the target was observed from a position downstream on the beam axis as shown in figure 2. The ion beam was deflected by a magnet onto a Faraday cup, after passing through the target to avoid damage to the crystal. Typical beam currents were 400 nA.

. * - - -

-. ^-.

^/ X-ray film Rowland

nagner fir r letu

'.

^\

I

/

~ a h e t

\

Faraday cup

Figure 2. Schematic diagram of experimental apparatus.

Figure 3. shows a spectrum of the 1s2 'So - ls2p 'PI, 3P1 transitions in Si12+, taken with a P E T 00% crystal (2d18 = 8.7358 f 0.0006

A)(2),

bent to a radius of 1291 mm. The exposure time was around 15 hours, and the total integrated beam charge was 21.6 mC. The observed resolving power is X/6X

-

^{3300. A} calculated rocking curve for PET 002 a t the Bragg angle of interest, 45", for X = 6.2

A

(for blue-shifted lines in Si12+) gives a theoretical resolving power X/6X

-

6000. Adding this in quadrature with the Doppler width expected from the discussion above agrees approximately with the observed linewidth. These transitions have been observed many times in laboratory beam- foil e ~ ~ e r i m e n t s ( ~ ~ ~ ) , in spectroscopy of the solar c o r ~ n a ( ~ ~ ~ ) , and of Tokomak plasmas(7), but this represents the highest resolution yet achieved.

-

Figure 3. Spectrum of SiI2f in the region of the l s 2 1 S - l s 2 p 'PI, PI transitions a t 6.6482Aand 6.6882A.

Figure 4. shows a spectrum of the Ne9+ Lyman a doublet, taken with the apparatus described previously, but with a K A P 002 crystal, (001 in second order, 2d=13.3164 A(')), in the spectrometer bent to a radius of 1073 mm. A calculated rocking curve for this crystal gives a theoretical resolving power of 7000. Adding this in quadrature with the expected Doppler broadening reproduces the observed resolving power, X/6X

-

^4800,

very well. T h e typical beam current during this experiment was 100 nA of Nee+ a t 39 MeV. T h e reduced beam current and low reflectivity of KAP in second order required a n exposure time of about 30 hours. T h e measured fine structure, corrected for t h e Doppler shift calculated from the accelerator operating conditions, and allowing for energy loss in the foil is 5.55 f 0.25 mA, which agrees with the theoretical prediction of 5.408 mA(').

Possible sources of error in our result come from satellite contamination of the spectrum, uncertainties in t h e beam ve!ocity and hence the blue shift, and Stark shifts induced by the wake field experienced by the ions within t h e foil.

T h e transitions we observe have, of course, large Doppler shifts, and so a precise knowledge of the ion beam velocity is required for any calibration of the spectrum with respect t o a stationary source. However, if suitable lines may be found within t h e beam itself for calibration(lO), precision measurements of transition wavelengths may be made without having t o measure the ion beam velocity. Alternatively, one may observe suitable targets from both upstream and downstream, in such a manner t h a t first order Doppler shifts can b e cancelled between the two spectra, leaving only the second and higher order effects t o be corrected for.

JOURNAL DE PHYSIQUE

Blue Shifted Wavelength

(A).

^--t

Figure 4.

Spectrum of the I s 2S1/2 - 2p 2P1/2,3/2 transitions in Ne9+.

References

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(5). A.B.C.Walker and H.R.Rugge, Astron. and Astrophys., 5 (1970) 4.

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