Abstract
The MOSCAB equipment, a geyser-concept bubble-chamber originally thought for the search of dark matter in the form of WIMPs, is employed for the detection of fast neutrons. Once the background-free operating conditions are determined such that the detector is sensitive only to neutrons, which occurs when the neutron energy threshold required for nucleation is higher than approximately 2.5 MeV, the detector response to fast neutrons is investigated using a {^{241}}AmBe neutron source. Sets of detection efficiency functions are then produced via Monte Carlo simulations and post-processing, their validation being performed experimentally and discussed. Finally, the use of the detector to measure the fast neutron activity of very weak n-sources in low neutron background environments, as well as to monitor the cosmic ray variations through the neutron component of the Extensive Air Showers, is considered.
Highlights
Once the background-free operating conditions are determined such that the detector is sensitive only to neutrons, which occurs when the neutron energy threshold required for nucleation is higher than approximately 2.5 MeV, the detector response to fast neutrons is investigated using a 241AmBe neutron source
The apparatus was moved underground to the INFN Gran Sasso National Laboratories (LNGS), repeating both background and neutron sensitivity measurements, which were performed for two different configurations of the detector, equipped with either a 2 L vessel filled with 1.2 L of C3F8 or a 18 L vessel filled with 13 L of C3F8
3.71 × 10−2 3.24 × 10−2 2.50 × 10−2 1.95 × 10−2 1.90 × 10−2 1.58 × 10−2 1.24 × 10−2 8.30 × 10−3 6.11 × 10−3 1.20 × 10−3 2.39 × 10−4 sitive only to neutrons. This occurs when the reduced superheat parameter S H is kept lower than 0.07, or the critical energy Ec is kept higher than nearly 200 keV, which approximately corresponds to a 2.5 MeV neutron energy threshold required for nucleation
Summary
The MOSCAB bubble-chamber detector relies on the geyser technique, a variant of the superheated liquid technique of extreme simplicity originally introduced by Hahn and Reist to detect fission fragments [1]. Since the critical energy must be released within a sufficiently small volume of the sensitive liquid, bubble nucleation depends on the stopping power of the ionizing particle, which makes the detector entirely unaffected by the backgrounds due to recoiling electrons and minimum ionizing radiation, provided that the critical energy is not too low. This means that, besides neutrons, the only other particleinduced background is represented by α-decay events, yet, as it will be shown, such a source of background can be completely suppressed by operating the detector above specific recoil energy thresholds. The detector sensitivity and its possible applications in measuring and monitoring low fluxes of fast neutrons in the absence of background are discussed
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