Short-range, high-LET recoil tracks in CR-39 plastic nuclear track detector E. R. Benton 1, C. E. Johnson 1, J. DeWitt 1, N. Yasuda 2, and E. V. Benton 3 1 Dept. of Physics, Oklahoma State University, Stillwater, OK USA 2 National Institute of Radiological Sciences, Chiba, Japan 3 Dept. of Physics, University of San Francisco, San Francisco, CA USA

Integral LET Fluence Spectra 60, 230, & 1000 MeV Protons on CR-39 PNTD CR-39 PNTD: 36 hr etch in 50°C, 6.25 N NaOH, B =~8  m

Proton- & Neutron-Induced Target Fragment Reactions

Contribution to Dose and Dose Equivalent from Proton-Induced Target Fragmentation LLUMC Proton Therapy Beam, Bragg Plateau Contribution of high LET (  5 keV/  m) particles to dose ranges from 3 to 4.5% The contribution of high LET particles to the dose equivalent ranges from ~25 to ~35% Only includes fragments with range >8 m ... shorter range fragments are not included.

CR-39 PNTD Exposures for AFM Analysis 10 µm x 10 µm CR-39 PNTD: 2-4 hr etch in 50°C, 6.25 N NaOH  B =  m Primary Proton Fluence: ~10 10 cm -2

3  cm -2 1 GeV Protons on CR-39 PNTD 4 hr etch, 1  m removed, 103  79  m 2

Short-Range, High-LET Recoil Tracks in CR-39 PNTD Light Microscopy Exposure: 10 8 cm MeV Protons 36 hr etch, 8  m removed 328  253  m 2 AFM Exposure: 1.5  cm -2 1 GeV Protons 4 hr etch, 1  m removed 103  79  m 2 No track >100 keV/  m For Comparison: 10 8 cm -2 1 GeV/n Ti 4 hr etch, 1  m removed 103  79  m 2 (~100 keV/  m)

4 hr etch, 1  m removed, 103  79  m  cm MeV Protons on CR-39 PNTD

Stopping Particles, Low Enery Scattering 1 GeV Protons on Au Target

Proton-induced Target Fragmentation as functions of Target Z and Energy CR-39 ( 12 C & 16 O) 27 Al 28 Si 64 Cu 108 Ag 197 Au 60 MeV (LLUMC) 230 MeV (LLUMC) 1 GeV (NSRL)

Target Fragment Fluence as functions of Proton Energy and Target

Fragment Distribution as a Function of Angle 230 MeV Protons (LLUMC)

Integral LET Fluence Spectra 230 MeV Protons on CR-39 PNTD AFM CR-39 PNTD: 2 hr etch in 50°C, 6.25 N NaOH, B = 0.5  m

Integral LET Fluence Spectra 1 GeV Protons on CR-39 PNTD AFM CR-39 PNTD: 2 hr etch in 50°C, 6.25 N NaOH, B = 0.5  m

Conclusions from Light Microscopy Analysis of CR-39 PNTD AFM Exposures The heavy recoil tracks often take the form of long tubes, not short cones. Tube-shaped tracks are characteristic of extremely high LET (>1000 keV/  m) particles near the end of their ranges. Some tracks “zig zag” toward their ends, illustrating range straggling. Some tracks are “forked”, illustrating low-energy Rutherford scattering. The size of the tracks (width and length) scales with Z of the target material. Fluence (and production cross section) scales with proton energy. Preliminary measurements appear to show that the angular distribution of high Z recoil fragments is isotropic.

Relevance/Applications Nuclear Physics: Validate Nuclear Reaction Models Provide Data to Extend Radiation Transport Codes Proton and Heavy Ion Therapy Radiation Effects on Electronic Circuits Space Radiation Dosimetry Study of Neutron Interactions with Heavy Nuclei

Conclusions Tracks from proton- and neutron-induced short-range (<20  m), high-LET recoil fragments in CR-39 PNTD are visible through a standard optical microscope for Bulk Etch of  m. New method to quantify heavy recoil nuclear processes previously not accessible due to short range of fragments. LET spectra measured by AFM consistent with previous LET measurements made by standard light microscopy. Extend work to other targets, different proton energies, heavy ions, and medium and high energy neutrons.