l. rymell, m. berglund, and h. m. hertz- debris-free single-line laser-plasma x-ray source for...

Debris-free single-line laser-plasma x-ray source for microscopy L. Rymell, M. Berglund, and H. M. Hertz a) Department of Physics, Lund Institute of Technology, P.O. Box 118, S-221 00 Lund, Sweden Received 19 January 1995; accepted for publication 3 March 1995 A debr is-f ree, narr ow-ba ndwid th, sing le-li ne lase r-p lasma soft x-ra y sour ce has been devel oped suitable for water-window microscopy using zone-plate optics. The 10 Hz table-top source utilizes microscopic ammonium hydroxide droplets as target. The N VI emission at 2.88 nm is 110 12 photons/ sr pulse. As an alternative to ammonium hydroxide, it is demonstrated that urea dissolved in water also produces strong emission from nitrogen ions, thereby extending the applicability of the dropl et x-ray source to a new range of target mat erials. © 1995 American Institute of Physics. In soft x-ray microscopy, the natural contrast betwen car- bon e.g., proteins and oxygen water in the water window 2.3–4.4 nm is utilized to allow high-resolution imaging of, e.g., unstained biological material in an aqueous environ- ment. Imaging and scanning soft x-ray microscopes based on Fresnel zone plate optics require a narrow-bandwidth, high- brightness soft x-ray source. 1– 3 Single-line emission and low continuum background is important to avoid image degrada- tion. Emission in the lower wavelength range of the water window is advantageous since it minimizes water absorption. In this letter we show that laser-produced plasmas in ammonium hydroxide and urea-water droplets provide a table-top soft x-ray source with these properties. To date, most work on x-ray microscopy is performed using synchrotron radi atio n sources. 1– 3 Noncompact laser- plasma sources have been used for flash contact microscopy 4 and x-ray laser imaging microscopy. 5 However, table-top mi- crosc opes show promise for sing le-s hot expos ures and in- creased acces sibi lity for appli ed resea rcher s. Exper iments wit h tab le- top sou rce s inc lud e the use of a nit rogen-gas plasma focu s 6 for water -win dow imag ing micr oscop y , 7 a carbo n-ta rget lase r plas ma 8 for wat er -wi ndo w sca nni ng microscopy 9 and la ser- pl as ma for hi gher -wavel engt h microscopy. 10 The high brig htness, smal l size, and spati al stability make the laser-plasma sources attractive. Using con- venti onal metal tar gets, conve rsion effici encie s of sever al tens of % may be reached wit h las er intensities of 10 14 W/cm 2 . 11 However, with solid state targets, the emission of debris may damage fragile x-ray optics positioned close to the pla sma. A sho rt dis tan ce bet wee n the plasma and the compon ent is impor tan t to inc rease the use ful x-r ay flux since the plasma is an incoherent x-ray source. We have pre- viously shown that debris is practically eliminated by using microscopic ethanol droplets as tar get. 12,13 Wi th this target liquid, multiline emission from C V and C VI ions in the upper wavelength half of the water window is produced. Be- low we describe the use of ammonium hydroxide droplets as a target for 10 Hz debris-free generation of N VI 2.88 nm and N VII 2.48 nm radiation in the lower part of the water window. With proper filtering, a high-brightness, singl e-li ne, narr ow-bandwidth sourc e at 2.88 nm wi th low continuum background is produced. To our knowledge, there is only one previous investigation of x-ray sources for x-r ay microscopy bas ed on las er pla sma s with a nit rogen target. In Ref. 6, a solid boron nitride target was illuminated by a 1 pulse/10 min J Nd laser resulting in, beside the debris issue, signi ficant broadband emissio n super impo sed on the N VI and V VII line emission. Boron nitride has also been used as a targ et for spectrosco pic inve stig ation s. 14 Other soli d tar get mate rial s, such as nitr ogen- rich organic salt s e.g., urea, have to our knowledge, not been investigated. In this letter, we show that practically debris-free droplet target operation can be extended to this class of materials by dis- solving them in a suitable liquid. The exper imen tal arra ngement for the ammo nium hydroxide droplet-target soft x-ray source is shown in Fig. 1. Concentrated ammonium hydroxide 32% NH 3 in water by volume at a pressure of 40 atmospheres is forced through a piezoelectrically vibrated glass capillary nozzle, resulting in a stable train of 10 m diam droplets in the 10 4 mbar pre ssu re vac uum cha mbe r . The noz zle pro duc es 10 6 drops/s with a droplet velocity of 50 m/s. A 50 mm focal le ngth le ns focuses the be am fr om a 10 Hz fr eque ncy- doubled, acti ve/ac tive/ passi ve mode locke d Nd:Y AG lase r Continuum PYC61-C to a FWHM of 12 m. The 100– 120 ps pulses have an energy of 70 mJ, resulting in a focal spot intensity of 510 14 W/cm 2 . The small size and high speed of the droplets in combination with the 12 m focal diameter makes spatial stability of the droplets and accurate temporal triggering of the laser important for a pulse to hit a single target droplet. Stable drop formation depends on the a Electronic mail: [email protected] FIG. 1. Exper imental arran gemen t for debris-f ree laser -pla sma soft x-ray generation with ammonium hydroxide droplet target. 2625 Appl. Phys. Lett. 66 (20), 15 May 1995 0 00 3- 69 51 /9 5/6 6( 20) /2 6 25 /3 /$ 6 .0 0 © 19 95 A me ri ca n I ns ti tu te o f P hy si cs Downloaded¬27¬Dec¬2004¬to¬130.237.35.30.¬Redistribution¬subject¬to¬AIP¬license¬or¬copyright,¬see¬http://apl.aip.org/apl/copyright.jsp

l. rymell, m. berglund, and h. m. hertz- debris-free single-line laser-plasma x-ray source for...

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