Calculation of low-energy neutron flux in the atmosphere by the S n method

We have calculated the global distribution of atmospheric neutrons and their products by a Monte Carlo simulation of nucleon transport, in the internuclear cascade followed by neutron transport below 19 Mev. First, we present the results generated by monoenergetic primary protons and alpha particles entering the top of the atmosphere. Second, the kernels derived from the monoenergetic cases are used to determine the spatial and energy distributions of neutrons and their products from the protons and alpha particles in the cosmic radiation; solar modulation effects are included. The calculation is compared, in the 1‐ to 10‐Mev region, with the results of our fast neutron experiment; the agreement is within the uncertainties of the primary spectrum and of the experimental results over most of the atmosphere. The calculation is then normalized to the experiment in the fast neutron region. The results of the normalized calculation include the steady state neutron spectrum, the neutron production rates, the radiocarbon production rates, the neutron leakage rates from the top of the atmosphere, and the production rates of other nuclides. The normalized calculation reproduces experimentally observed slow neutron densities and the observed neutron flux and spectrum above 1 kev, and it predicts features of the atmospheric neutron morphology not yet observed. The points of agreement and divergence with earlier calculations are discussed, including the radiocarbon production rates and the neutron leakage rates during solar cycle 20, which is near the mean of the last 10 solar cycles.

Section: Comparison With Other Calculationsmentioning

confidence: 99%

Time dependent worldwide distribution of atmospheric neutrons and of their products: 2. Calculation

Light¹,

Merker²,

Verschell³

et al. 1973

“…The more recent calculations of Wilson et al [1969], in which some structure in the spectrum is evident, fit the June data possibly better than the Newkirk calculations. The Wilson calculation, however, (Table 3) total neutron leakage flux is obtained using the Newkirk [1963] spectrum (Table 4). For the equatorial regions the Newkirk spectral shape agrees with our measurements.…”

Section: Equatorial Region Where Po Is •12 Gvmentioning

confidence: 99%

“…The efficiency is plotted in Figure 1 [Newkirk, 1963;Lingen[elter, 1963] were used to deduce mean efficiencies for the 8I-Ie counter [Jenkins et al, 1970]. From these efficiencies the observed counting rates (GN) can be converted to a total neutron leakage flux.…”

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confidence: 99%

“…two spectra. The ratio of fast neutron leakage flux to total leakage flux thus found from the scintillator and SHe-counter rates was then compared with the ratios predicted directly from the Lingen•elter[1963] andNewkirk [1963] spectra.For the Lingenfelter spectrum, the predicted ratio of fast neutron leakage flux to total neutron leakage flux near the poles is 0Differential neutron energy spectra. The Lingenfelter and Newkirk spectra have been normalized to the Ogo 6 measured neutron flux for E < 1 Mev.…”

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confidence: 99%

See 1 more Smart Citation

The energy dependence of the cosmic-ray neutron leakage flux in the range 0.01-10 Mev

Jenkins¹,

Ifedili²,

Lockwood

et al. 1971

The cosmic‐ray neutron leakage flux and energy spectrum in the range 1–10 Mev were measured by a neutron detector on the Ogo 6 satellite from June 7 to September 30, 1969. The same detector simultaneously measured the total leakage flux, having 75% of its response to the leakage flux in the interval 1 kev to 1 Mev. For a neutron energy spectrum of the form AE−γ in the range 1–10 Mev, the upper limit to γ for polar regions (Pc⪕0.3 Gv) was found to be 1.0 and for the equatorial regions ( Pc⪖12 Gv) was 1.2. For the polar regions, the lower limit to γ was found to be 0.8. This energy spectrum at 1–10 Mev is slightly flatter than L. L. Newkirk predicted. The neutron fluxes at 1–10 Mev were 0.28±0.03 and 0.035±0.003 cm−2 sec−1 for Pc⪕0.3 and Pc⪖12 Gv, respectively. The ratios of the neutron flux at 1–10 Mev to the total neutron flux were 0.54±0.07 for Pc⪖0.3 Gv and 0.50±0.06 for Pc>12 Gv.

“…[1966] point but that their measured neutron production at sea level is also about a factor of three smaller than the neutron production.predicted from the data of Hess et aZ. [1959, 19611 andNewkirk [1963]. O'Brien [1971] calculated the depth dependence of the neutron flux' in the atmosphere using an essentially analytical propagation theory and a phenomenological in Fig.…”

Section: Density-lependentmentioning

confidence: 99%

Calculations of neutron flux spectra induced in the Earth's atmosphere by galactic cosmic rays

Armstrong¹,

Chandler²,

Barish³

1973

165

109

Calculations have been carried out to determine the neutron flux induced in the earth's atmosphere by galactic protons and alpha particles at solar minimum for a geomagnetic latitude of 42°N. Neutron flux spectra in the energy range ∼10−8 to ∼105 Mev at various depths in the atmosphere were calculated by using Monte Carlo and discrete ordinates methods, and various comparisons with experimental data are presented. The magnitude and shape of the calculated neutron leakage spectrum at the particular latitude considered support the theory that the cosmic ray albedo neutron decay (Crand) mechanism is the source of the protons trapped in the inner radiation belt.