21-29 July 2022
US/Pacific timezone

Measurement of the $^{35}$Cl radiative neutron capture cross section at the n_TOF facility, CERN

25 Jul 2022, 09:54
Folsom ()



Sam Bennett The n_TOF Collaboration


The $^{35}$Cl radiative capture rate is important in a number of applications. The long-lived radionuclide $^{36}$Cl is a by-product of the activation of $^{35}$Cl present in graphite moderated reactors both in the fuel cladding and as an impurity in nuclear graphite (< 2 ppm by mass). Reliable predictions of the amount of $^{36}$Cl present in the large volume of irradiated nuclear graphite waste relies on accurate reaction cross section data, essential for its safe disposal [1]. Moreover, in Boron Neutron Capture Therapy (BNCT), currently being considered more widely as a cancer therapy [2], accurate knowledge of the dose rate delivered both to tumours and the surrounding healthy tissue is imperative; chlorine has an important role in the dose rate, especially in brain tissue [3] where it represents 0.3% by mass. Simulations have indicated that around 11% of the total dose rate relevant to the neutron energy ranges used in BNCT (100 eV - 10 keV) comes from neutron capture on 35Cl. Finally, $^{35}$Cl is one of several ‘minor neutron poisons’ in the astrophysical s-process, reducing the efficiency of neutron recycling and is furthermore involved in the as yet unknown origin of $^{36}$S [4, 5]; accurate capture cross section data are important in determining the significance of $^{35}$Cl in the s-process and its impact on stellar reaction networks.
The reaction cross section has been measured twice via the time-of-flight method in the past [6, 7], for which the results of resonance analyses are discrepant by around 15%, and evaluations (ENDF/B-VIII.0, JEFF-3.3, JENDL-4.0) differ with respect to one another by around 10% in the resonance region. The recent AMS measurement of the 30 keV Maxwellian averaged cross section is inconsistent with the existing time-of-flight measurements [8].
Work has been performed to accurately measure the $^{35}$Cl$(n,\gamma$)$^{36}$Cl reaction cross section at the 185m beam-line at the neutron time-of-flight facility (n_TOF) at CERN, using a C6D6 total energy detection setup. We measured thirteen resonances up to a maximum energy of around 50 keV, limited to the strongest resonances by a prohibitive background. Preliminary results are in agreement with the results of Reference [6], and the Maxwellian averaged cross section extracted from the data is consistent with the recent AMS measurement [8]. Our results indicate that the ENDF/BVIII.0 and JEFF-3.3 evaluations (both based on the R-Matrix analysis of Sayer et al. [9]) underestimate the cross section by around 15%. The experimental procedure, analysis and results of this measurement shall be presented.
[1] R. Mills, Z. Riaz, A. Banford, Nuclear Data issues in the calculation of 14C and 36Cl in irradiated graphite, ENC 2012 Conference proceedings.
[2] International Atomic Energy Agency (IAEA), press release: https://www.iaea.org/newscenter/news/boron-neutron-capture-therapy-back-in-limelight-after-successful-trials
[3] R. F. Barth, M. G. H. Vicente, O. K. Harling et al., Current status of boron neutron capture therapy of high grade gliomas and recurrent head and neck cancer, Radiation Oncology 7, 146 (2012).
[4] H. Schatz, S. Jaag, G. Linker, R. Steininger, F. Käppeler, P. E. Koehler, S. M. Graff, and M. Wiescher, Phys. Rev. C 51, 379 (1995). 
[5] R. Reifarth, K. Schwarz, and F. Käppeler, Astrophys. J. 528, 573 (2000). 
[6] R. L. Macklin, Phys. Rev. C 29, 1996 (1984). 
[7] K. H. Guber, R. O. Sayer, T. E. Valentine, L. C. Leal, R. R. Spencer, J. A. Harvey, P. E. Koehler, and T. Rauscher, Phys. Rev. C 65, 058801 (2002). 
[8] S. Pavetich, A. Wallner, M. Martschini et al., Accelerator mass spectrometry measurement of the reaction $^{35}$Cl$(n,\gamma$)$^{36}$Cl at keV energies, Phys. Rev. C 99, 015801 (2019).
[9] R. O. Sayer, K. H. Guber, L. C. Leal et al., R-matrix analysis of Cl neutron cross sections up to 1.2 MeV, Physical Review C 73, 044603 (2006)

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