Gunst, J.

Keitel, C. H.

Litvinov, Yury

Palffy, A.

GSI Repository

Dominant secondary nuclear photoexcitation with the XFELJ. Gunst∗1, Y. A. Litvinov†2, C. H. Keitel‡1, and A. Pa´lffy§11Max-Planck-Institut fu¨r Kernphysik, Heidelberg, Germany; 2GSI, Darmstadt, GermanyPhoton energies accessible with the new x-ray free-electron laser (XFEL) facilities open the way for the res-onant driving of nuclear transitions. In particular, thehigh brilliance and coherence features of the XFEL lightpromise an increase in nuclear excitations in compari-son to synchrotron radiation experiments. While in thelatter experiments the electronic response only acted asbackground, the unique interaction between high-intensityXFEL pulses and matter can lead to new states, like cold,high-density plasmas [1] where secondary nuclear pro-cesses from the coupling to the atomic shell are renderedpossible. In this work we show on the example of 93mMoisomer triggering that secondary nuclear excitation by elec-tron capture (NEEC) can even dominate the direct resonantphotoexcitation.The idea of isomer triggering is to connect themetastable isomeric state to an above-lying level in orderto release the stored excitation energy on demand [2]. The6.85 h long-lived 93mMo isomer is particularly attractivefor an XFEL-induced activation, since the 2.5 MeV ex-citation energy can be retrieved by a 4.85 keV triggeringtransition accessible by todays XFELs. In the subsequentdecay cascade an outstandingly high energetic photon of1 MeV can be used as distinct signature for the nuclear ex-citation. Moreover, in 9341Nb(p,n)93m42Mo reactions, the iso-mers can be produced directly embedded into 1 µm thickniobium foils, providing high-density targets. Using, forinstance, the LINAC proton beam at GSI a density of 1016isomers/cm3 can be achieved.The direct light-nucleus coupling between the XFEL ra-diation and the Nb target foils can be described by thedensity matrix formalism using the semiclassical approach.Although the XFEL pulse is tuned on the 4.85 keV trigger-ing transition, only a small fraction of the laser photons ful-fill the nuclear resonance condition due to the large discrep-ancy between the laser width ΓXFEL (∼10 eV) and the nu-clear transition width Γn (∼10−7 eV). Taking this effectiveintensity reduction into account and considering laser pa-rameters for the LCLS at SLAC a signal rate of 5.6×10−14photons/s is obtained [3]. A further analysis shows that thenuclear photoexcitation is strongly suppressed by the poortemporal coherence of current XFEL facilities.The nuclear excitation induced directly by the lasershould be compared to its secondary electronic-processes-induced counterpart. The XFEL-induced plasma forma-∗ Jonas.Gunst@mpi-hd.mpg.de† Y.Litvinov@gsi.de‡ Keitel@mpi-hd.mpg.de§ Palffy@mpi-hd.mpg.de0 1000 2000 3000 4000Kinetic electron energy [eV]10−810−610−410−2100102104NEECcrosssection[b]Capture into:L shellM shellN shellO shell1029103010311032103310341035 Electrondistribution[1/m2/s/eV]El. distribution:Te=350eVFigure 1: NEEC resonance cross sections (left axis) to-gether with the electronic energy distribution (right axis).tion is governed by inner-shell photoionization and sub-sequent refilling by either radiative or Auger decay. Dueto the fast and uniform heating process the plasma staysalmost at solid-state density. After the irradiation of thesample, radiative and collisional processes lead to a fastthermalization among the electrons. In our scenario, weestimate an electron temperature Te and density of 350 eVand 1.3×1024 cm−3, respectively, and consider the plasmaconditions to hold for 100 ps.In this environment, NEEC takes place on a longer timescale compared to the laser pulse duration. The total NEECrate in the plasma relies on the available charge states andelectron energies. As shown in Fig. 1, NEEC prefers thecapture into deep vacancies. At Te=350 eV, the L shellseems to provide the pivotal NEEC channels. However,the available charge states are limited to q=18+. . .30+ pro-hibiting the capture into the L shell. Nevertheless, a sum-mation over all open resonance channels results in a NEEC-induced signal rate of 1.7× 10−7 photons/s which is 6 or-ders of magnitude higher than the direct process [3]. Ad-ditionally, NEEC represents the more robust mechanismsince it is less sensitive to the match between the photonfrequency and the nuclear resonance condition.References[1] S. Vinko et al., Nature 482, 59 (2012).[2] A. Pa´lffy et al., Phys. Rev. Lett. 99, 172502 (2007).[3] J. Gunst et al., Phys. Rev. Lett. 112, 082501 (2014).GSI SCIENTIFIC REPORT 2013 PNI-AP-12doi:10.15120/GR-2014-1-PNI-AP-12 169

Dominant secondary nuclear photoexcitation with the XFEL

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Dominant secondary nuclear photoexcitation with the XFEL

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