Precision imaging of 4.4 MeV gamma rays using a 3-D position sensitive Compton camera

Ayako Koide,Koki Sueoka,Jun Kataoka,Leo Tagawa,Takamitsu Masuda,Takuya Maruhashi,Kazuya Fujieda,Takanori Taya,Takuya Kurihara,Saku Mochizuki,Taku Inaniwa

doi:10.1038/s41598-018-26591-2

Abstract

Imaging of nuclear gamma-ray lines in the 1–10 MeV range is far from being established in both medical and physical applications. In proton therapy, 4.4 MeV gamma rays are emitted from the excited nucleus of either 12C* or 11B* and are considered good indicators of dose delivery and/or range verification. Further, in gamma-ray astronomy, 4.4 MeV gamma rays are produced by cosmic ray interactions in the interstellar medium, and can thus be used to probe nucleothynthesis in the universe. In this paper, we present a high-precision image of 4.4 MeV gamma rays taken by newly developed 3-D position sensitive Compton camera (3D-PSCC). To mimic the situation in proton therapy, we first irradiated water, PMMA and Ca(OH)2 with a 70 MeV proton beam, then we identified various nuclear lines with the HPGe detector. The 4.4 MeV gamma rays constitute a broad peak, including single and double escape peaks. Thus, by setting an energy window of 3D-PSCC from 3 to 5 MeV, we show that a gamma ray image sharply concentrates near the Bragg peak, as expected from the minimum energy threshold and sharp peak profile in the cross section of 12C(p,p)12C*.

Highlights

Gamma ray imaging techniques are widely used in various fields of nuclear medicine, industries, nuclear and elementary particle physics, and in high energy astrophysics
The presented spectra are corrected for efficiency of high-purity Germanium (HPGe) detector, which decreases as the incident gamma ray energy increases
Various nuclear emission lines were observed in the prompt gamma ray spectra shown in Fig. 1 and Table 1, it is still controversial which line or energy band is most suitable for online proton therapy monitoring

Summary

Introduction

Gamma ray imaging techniques are widely used in various fields of nuclear medicine, industries, nuclear and elementary particle physics, and in high energy astrophysics. Owing to the use of pair annihilation of 511 keV gamma rays, a PET scanner does not need a collimator and is widely used to find tumors and diagnose Alzheimer’ disease[4] In both imaging techniques, photo-absorption is the dominant process within a detector, a dense high speed scintillator is generally used. Very high energy gamma rays (greater than 10 MeV) are often an active research target in modern astronomy and elementary particle physics At such energies, the dominant interaction between photons in the material is pair production, particle tracking systems like silicon strip detectors (SSD5) or scintillation fibers are commonly used to determine the incident direction of gamma rays[6,7]. While the radioactive material distributed in the accident was mainly 137Cs, which emits 662 keV gamma rays, various attempts were made to visualize sub-MeV to MeV gamma-rays quickly and accurately with the most recent detector technologies[33,37,38,39,40,41,42]

Methods

Results

Conclusion