Pulsed-beam neutron-lifetime measurement

Abstract

We have measured the neutron lifetime using a superconducting electron spectrometer and a pulsed beam of cold neutrons. Spatially defined neutron bunches are completely contained within the spectrometer's active volume while the /3-decay rate is measured. The flux is determined from the radioactivity of nearly totally absorbing thick samples of cobalt and gold exposed to the neutron beam. We obtain r" =876 ± 21 sec.PACS numbers: 13.30.Ce, 14.20.Dh, 23.40.Bw The measured neutron lifetime provides information about fundamental parameters, in particular the vector and axial-vector weak-coupling constants, Gv and GA> Knowing these quantities is crucial for an understanding of the weak interaction and there are important consequences for cosmology and astrophysics. x At present Gv is best determined from nuclear /?-decay experiments and the experimentally determined neutron lifetime is used to infer GA/GVIn previous experiments electrons or protons were detected from neutrons decaying in continuous thermal or cold beams. Published lifetimes 2 vary between 877 and 937 sec, with errors between 8 and 18 sec. The discrepancies are much larger than the stated errors. Until recently, precise values for GA/GV came only from the experimental lifetime, but the inconsistencies introduce a large systematic uncertainty. Fortunately, a recent measurement of the neutron p asymmetry determines GA/GV better than the lifetime measurements, mitigating the difficult choice between inconsistent lifetimes. 3 Nevertheless, it is important to resolve the lifetime discrepancies. The combination of very precise neutron-decay experiments will eventually allow us to determine both GA and Gv without resorting to more complex nuclear systems.We measured the neutron lifetime with a new method using a pulsed neutron beam. This pulsed beam passes through an "in-beam" electron spectrometer with large active volume. We detect decay electrons during short time intervals of length At ~ 0.5 msec, when neutron bunches are fully contained within the spectrometer and all neutron decays with electron energies above threshold have the same detection probability. The neutron lifetime r n is obtained from the decay law AN n /At = -N n /T n . The integrated number of neutrons N n comes from measurements of the y activity, n r =N n /r r , of thick samples of high-purity 5 Co and 197 Au exposed to the beam. The neutron-decay rate ANjAt is obtained from the measured p rate n p =Np/At. Thus the experimental method is summarized by the simple relation Tn^tyiriy/np) which relates the neutron lifetime to the well-known nuclear lifetimes x y .Compared to continuous-beam lifetime measurements our method does not require precise knowledge of the effective length of the spectrometer or the neutron velocity distribution. In many previous experiments sensitivity to the neutron velocity spectrum is avoided by the measurement of the neutron-capture flux directly with thin detectors containing 3 He or 10 B. However, this procedure relies on the exactness of the l/v dependenc...

A clean, bright, and versatile source of neutron decay products

“…[23]. We have tested the principle of n-decay from a propagating neutron cloud earlier with a free n-beam [26]. used, which at present is (99.770.1)% at 10% polarizer transmission [27] (relative to the incoming unpolarized n-flux).…”

Performance of the Los Alamos National Laboratory spallation-driven solid-deuterium ultra-cold neutron source

“…[1][2][3] In the past decade, experiments have demonstrated several advantages of using ultra-cold neutrons for beta decay experiments. Measuring the decay of neutrons trapped in material bottles eliminates systematic errors associated with defining the volume and flux that have plagued neutron lifetime experiments with cold neutron beams [4][5][6][7] and therefore allows more precise measurements. [8][9][10][11][12][13] More recently, results obtained with the Los Alamos Neutron Science Center (LANSCE) UCN source 14,15 have demonstrated that the high polarizations and low backgrounds that can be obtained with spallation-driven, pulsed UCN sources can reduce the systematic errors in measuring the spin dependence of neutron beta decay 16,17 relative to cold neutron beam-based experiments.…”

12 The Measurement of Nuclear Lifetimes

“…Figure 2 and Table 1 summarize the present status of the heaviest elements known and their production and decay properties [20,21] 54 Cr, 58 Fe) and subsequent evaporation of 1-2 neutrons. The fragments decay within some 10~4-ls, mainly via the emission of 9-11 MeV ot-particles and (possibly) fission.…”

“…In the experiment by Last et al [58], the neutrons in the pulse are counted by a cobalt beam stop, i.e. by activating a 59 Co sample and precisely measuring the decay γ-radiation.…”