New Developments in Proton Radiography at the Los

Authors: C L Morris E N Brown C Agee T Bernert M A M Bourke M W Burkett W T Buttler D D Byler C F Chen A J Clarke J C Cooley P J Gibbs S D Imhoff R Jones K Kwiatkowski F G Mariam F E Merrill M M Murray C T Olinger D M Oro P Nedrow A Saunders G Terrones F Trouw D Tupa W Vogan B Winkler Z Wang M B Zellner

Publish Date: 2015/12/30

Volume: 56, Issue: 1, Pages: 111-120

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Abstract

An application of nuclear physics a facility for using protons for flash radiography has been developed at the Los Alamos Neutron Science Center LANSCE Protons have proven far superior to high energy xrays for flash radiography because of their long mean free path good position resolution and low scatter background Although this facility is primarily used for studying very fast phenomena such as high explosive driven experiments it is finding increasing application to other fields such as tomography of static objects phase changes in materials and the dynamics of chemical reactions The advantages of protons are discussed data from some recent experiments will be reviewed and concepts for new techniques are introducedIn the mid1990’s a new tool to aid in the mission of stewarding the US nuclear stockpile was invented at Los Alamos namely proton radiography pRad 1 The concept uses the attenuation due to the nuclear scattering of very short pulses of energetic protons as they transit high explosive driven experiments to provide contrast for flash radiography The long mean free path of intermediate energy 10’s of GeV protons mitigates many of the difficulties encountered over the previous 5 decades of flash radiograph with high energy Xrays such as large scatter backgrounds low dynamic range and poor position and temporal resolution 2 3Experiments preformed using 24 GeV protons provided by the Alternating Gradient Synchrotron AGS accelerator at Brookhaven Nation laboratory demonstrated all of the expected gains from pRad when compared to Xrays with a set of unclassified 4 and classified static objects 5 The use of pRad for dynamic testing of surrogate nuclear weapon primaries is being pursued by both the Russian 3 6 and Chinese 7 8 9 10 11 weapons program but not by the United StatesHowever a proton flashradiography facility that uses the 800 MeV beam from the Los Alamos Neutron Science Center LANSCE at Los Alamos National Laboratory has become a work horse for the US stockpile stewardship program for smaller experiments studying the science of explosively driven systems A recent review describes the techniques of proton radiography and presents some experimental results 2The interaction of energetic protons with matter is governed to high precision by nuclear and Coulomb interactions Protons lose energy to the matter because of the Coulomb scattering of the protons from the atomic electrons and they scatter from the nuclei both because of the strong interaction and the Coulomb interaction with the proton For the purposes of understanding proton radiography these interactions can be factorized and treated independently The energy loss is given by the BetheBloch model 12 Coulomb scattering is given by the Moliere theory 13 14 and nuclear scattering can be described by the black disk or optical model of nuclear scattering 15Proton radiography is performed by illuminating a target object with a beam of protons and then by focusing the transmitted protons onto a scintillator screen 16 17 using a quadrupole magnetic lens 18 19 A collimator located at the Fourier plane of the lens is used to control the amount of contrast produced by Coulomb scattering Nuclear interactions scatter or absorb beam particles generally to angles far outside the multiple scattering cone which is on the order of 10 mrad in typical LANSCE experiments The formalism used for analyzing pRad data is described in reference 2 and a summary of the physics listed above can be found in the particle data review 20The absorption cross section σ A for hadrons on a nucleus with mass number A is often approximated by σ A  = πr A 2 accurate to ≈20  the geometric cross section of the nucleus where r Aapprox 12Afrac13 fm Here p is the proton momentum β is its velocity relative to the speed of light and X 0 is the radiation length 21 For monolithic materials the proton radiography transmission equation can be inverted to obtain the thickness of an object For objects where zλ one can obtain radiation weighted thicknesses by solving equation 1 Because of generally small backgrounds precise thicknesses on the order of one percent can be obtained from proton radiography in these cases 22

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