Soft Report - Dipartimento di Fisica - Sapienza

Scientific Report – Elastic and inelastic scattering of neutrons and X-raysThe Dynamics of Dilute H 2 Enabling New Calibration Methodsin Neutron SpectroscopyThe never fading scientific interest in the simplestand most fascinating molecular system, fluid H 2, haslately been revived by the possibility of convertingthe knowledge of its dynamic response to slow andthermal neutrons into a powerful technique for datanormalization in inelastic neutron scatteringexperiments. This possibility was, until very recently,IH2 [arb. units]α0 ≈ 30’a)6 x 10-3 E f = 50 meV5θ = 2°4321E [meV]0-20 -15 -10 -5 0 5 10b)c)α1 = 40’MonochromatorCu(111)Monitorθ MSamplej-th cell:θ j , ∆Ω j , ε jSlitsα2 = α1θFilterα3 = 60’3 He DetectorAnalyzerCu(111)/PG(002)i-th cell:θ i , ∆Ω i , ε iFig. 1: a) Experimental setup; b) Neutron spectrumof dilute H 2 (blue dots) compared with semiclassical(green curve) and quantum (red curve) predictions;c) The big BRISP detector inside its long vacuumchamberat the ILL, ideally subdivided into a 2Darray of square detection cells (see text).θ Ahindered by the absence of an experimentalverification of available theoretical predictions for theH 2 roto-translational neutron spectra in the dilute,room temperature, phase.A successful attempt to overcome this lack ofinformation, carefully testing the dynamical modelsagainst first neutron data for low-density H 2, wasrecently performed [1] by means of three-axisspectrometry (IN3 instrument at ILL), using theinstrumental configuration shown in Fig. 1 a). Thedynamic response of dilute hydrogen was measuredat two fixed final neutron energies, namely E f = 14.7and 50 meV, and rather low scattering angles, i.e. inthe most demanding cases for theoreticalmodelization and, at the same time, in the mostuseful conditions for the setting up of a valuable andalternative method to the well-known vanadiumcalibration technique in neutron spectroscopy, whichcan lose accuracy at low momentum transfers. Adetailed data analysis of the H 2 neutron spectra,along with implementation of both semiclassical andquantum calculations of the expected intramoleculardynamics [1], distinctly showed the superiority of thequantum-mechanical models. An example is shownin Fig. 1 b). The agreement found between measuredand quantum-calculated spectra, makes H 2 aconvenient fluid reference sample, particularly suitedto small-angle experiments, and, more generally, toinvestigations where the similarity between thegeometrical configuration of a liquid sample insideacontainer and the employed normalization standardis crucial.The hydrogen calibration technique becomes evenmore powerful in the case of two-dimensionaldetection at small angles, as for the new BRISPspectrometer [2]. The availability of a referencesample characterized by a broad energy spectrumlike H 2, and the detailed knowledge of its scatteringlaw for each (E 0, E f, θ)-triplet, E 0 and θ being theincident neutron energy and scattering angle, allowsfor an accurate “cell-by-cell” normalization ofexperimental intensities. Individual normalizationfactors, depending on specific scattering angle θ,solid angle ∆Ω, and efficiency ε, can now be assignedto the sample intensities, for each E value and foreach detector element (see an example subdivisionin Fig. 1 c), with unprecedented accuracy.References[1] E. Guarini, A. Orecchini, F. Formisano, F.Demmel, C. Petrillo, F. Sacchetti, U. Bafile, and F.Barocchi, J. Phys.: Condens. Matter 17, 7895(2005).[2] D. Aisa et al., Nucl. Instr. Meth. A 544, 620(2005).AuthorsF. Barocchi, U. Bafile, F. Formisano, E. Guarini, A.Orecchini, C. Petrillo, F. Sacchetti,Dipartimento di Fisica, Università di Firenze CRS-SOFT, CNR IFAC, OGG Grenoble CRS-SOFT,Dipartimento di Fisica, Università di Perugia, CRS-SOFTSOFT Scientific Report 2004-06104