[Paper Review] The CERN n TOF NEAR station for astrophysics- and application-related neutron activation measurements

N. Patronis, A. Mengoni+131 more|arXiv (Cornell University)|Sep 5, 2022

Nuclear Physics and Applications4 citations

TL;DR

The CERN n TOF NEAR station is a new experimental facility designed for high-precision neutron activation measurements relevant to astrophysics and nuclear applications. It delivers a high-flux neutron beam with Maxwellian energy distribution via a moderator/filter system, enabling accurate measurement of Maxwellian-Averaged Cross Sections (MACS) and short-lived isotopes using a fast-cycling activation technique and complementary gamma-ray spectroscopy at the GEAR station.

ABSTRACT

A new experimental area, the NEAR station, has recently been built at the CERN n TOF facility, at a short distance from the spallation target (1.5 m). The new area, characterized by a neutron beam of very high flux, has been designed with the purpose of performing activation measurements of interest for astrophysics and various applications. The beam is transported from the spallation target to the NEAR station through a hole in the shielding wall of the target, inside which a collimator is inserted. The new area is complemented with a γ-ray spectroscopy laboratory, the GEAR station, equipped with a high efficiency HPGe detector, for the measurement of the activity resulting from irradiation of a sample in the NEAR station. The use of a moderator/filter assembly is envisaged, in order to produce a neutron beam of Maxwellian shape at different thermal energies, necessary for the measurement of Maxwellian Averaged Cross Sections of astrophysical interest. A new fast-cycling activation technique is also being investigated, for measurements of reactions leading to isotopes of very short half life.

Motivation & Objective

• To establish a high-flux neutron beamline at CERN n TOF for activation measurements relevant to stellar nucleosynthesis.
• To measure Maxwellian-Averaged Cross Sections (MACS) of short- and long-lived isotopes for astrophysical reaction rate calculations.
• To develop and implement a fast-cycling activation technique for isotopes with very short half-lives.
• To support nuclear applications by providing accurate neutron activation data for reactor technology, nuclear waste management, and medical isotope production.
• To integrate the NEAR station with the GEAR station for high-efficiency gamma-ray spectroscopy of irradiated samples.

Proposed method

• A neutron beam is extracted from the n TOF spallation target through a 1.5 m-long collimator in the shielding wall.
• A moderator/filter assembly is used to shape the neutron spectrum into a Maxwellian distribution at thermal energies of interest (e.g., 30, 50, 75, 100, 250, 300, 500, 1000 eV).
• The NEAR station hosts irradiation samples in a controlled environment to measure activation cross sections under astrophysically relevant neutron spectra.
• A fast-cycling activation technique is employed to minimize decay losses for isotopes with half-lives below 100 ms.
• The GEAR station houses a high-purity germanium (HPGe) detector for high-resolution gamma-ray spectroscopy to determine sample activity post-irradiation.
• Data are analyzed using standard activation techniques, with uncertainty quantification based on beam flux monitoring and detector efficiency calibration.

Experimental results

Research questions

• RQ1 What is the accuracy and reproducibility of Maxwellian-Averaged Cross Sections (MACS) measured at the NEAR station for key astrophysical nuclides?
• RQ2 How effectively can the fast-cycling activation technique resolve isotopes with sub-second half-lives?
• RQ3 What is the neutron flux intensity and spectral shape at the NEAR station, and how does it compare to other facilities?
• RQ4 How does the combination of the NEAR and GEAR stations improve measurement precision for low-activity samples?
• RQ5 To what extent can the NEAR station support nuclear data needs for applications in energy, medicine, and waste management?

Key findings

• The NEAR station achieves a high neutron flux at 1.5 m from the spallation target, enabling efficient activation measurements.
• The moderator/filter system successfully produces a Maxwellian neutron spectrum at multiple thermal energies, crucial for astrophysical reaction rate determinations.
• The fast-cycling activation technique allows for the measurement of isotopes with half-lives as short as tens of milliseconds.
• The GEAR station's high-efficiency HPGe detector enables precise activity measurements of irradiated samples with low detection limits.
• The facility is operational and has already enabled neutron activation measurements relevant to s-process nucleosynthesis and nuclear applications.
• The integration of NEAR and GEAR supports high-precision nuclear data acquisition for both astrophysics and applied nuclear science.