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Measurements of Neutron Spectra Induced by Cosmic Radiation at Altitudes of 85 m, 1195 m and 2650 m

04.01.2010

Between May 2005 and July 2006, the "Neutron Radiation" Department carried out three measurement campaigns for the determination of neutron spectra at altitudes of 85 m, 1195 m and 2650 m. The neutrons which were measured stem from the interaction between cosmic radiation and the molecules of the Earth's atmosphere.

Measurements of neutron spectra induced by cosmic radiation with NEMUS

Figure 1a : Reference measuring set-up for environmental radiation

Figure 1b : SIL: BfS’s (Federal Ministry for Radiation Protection) measuring station for atmospheric radioactivity on top of Schauinsland mountain

Figure 1c : UFS: Environmental research station Schneefernerhaus (2650 m)

The measurements were carried out with the extended-range Bonner sphere spectrometer NEMUS [1]. It consists of a total of 14 moderating spheres of polyethylene, four of which are equipped with additional metallic shells in order to increase the sensitivity at neutron energies above 20 MeV. 3He proportional counters are located inside the spheres in order to record thermal (i.e. slow) neutrons. In addition, a 3He proportional counter is operated without a moderating sphere.

Since the number density of the neutrons from cosmic radiation is very low, the measurement campaigns usually take 6 to 8 weeks. In order to obtain an averaged result from the data gained over the total measurement period, it is vital to correct for the changing air pressure and other environmental influences (such as, e.g., additional moderating in the event of snowfall). Furthermore, measurements with extreme electronic noise must be identified and discarded (e.g. microphonic events).

The NEMUS spectrometer makes it possible to carry out measurements with up to 8 spheres in parallel; for each of the spheres, the pulse height spectra (PHS) are recorded which have been measured over a period of 6 hours (3 hours for UFS). Especially at small pulse heights, some of the events are not caused by neutrons. In order to sort these events out, the measured PHS in the range of high channels are adjusted to a model spectrum. The model spectra are individual for each counting tube and are measured under laboratory conditions prior to the measurement campaign. In the end, the newly developed adjustment procedure provides a counting rate for each sphere which has been corrected for a previously defined reference air pressure. The details of the analysis, in particular the sphere-specific air pressure correction, have recently been presented at the international neutron dosimetry symposium NEUDOS-11 [2].

From the counting rates determined in this way, the differential neutron energy distribution (or in short: the neutron spectrum) is determined by means of a mathematical procedure called deconvolution. This procedure is based on a two-step process:

  1. The parameters for a suitably selected parameterised spectrum are determined by means of the Bayesian method [3, 4].
  2. The spectrum from the first step is used as pre-information (default spectrum) for the deconvolution with the computer program MAXED [5]. This program is based on the theory of the "maximum entropy". The results of the deconvolution are shown in Fig. 2.

Figure 2 : Neutron spectra induced by cosmic radiation at different altitudes.

The table lists the numerical results obtained for the neutron fluence rates  (integrated over the total energy range) as well as the ambient dose equivalent rate from the three measurement campaigns. Thereby, the dose rates were calculated by multiplying the deconvoluted spectra by the fluence-to-dose conversion coefficients from ICRP 74 [6]. The uncertainties given are taken from the Bayesian analysis.

LocationAltitude
m
Ambient
pressure
hPa

cm2h-1

nSvh-1
PTB85100448.8±2.111.2±1.1
SIL119588091.4±4.120.4±2.0
UFS2650735339±1269.0±5.5

Literature:

  1. B. Wiegel and A.V. Alevra:
    NEMUS The PTB Neutron Multisphere Spectrometer: Bonner Spheres and More
    Nucl. Instrum. Meth. A476, 36 41 (2002).
  2. A. Zimbal, A. Kasper, M. Reginatto, H. Schuhmacher and B. Wiegel:
    Measurements of Neutron Energy Spectra at Altitudes of 85 m, 1195 m and 2650 m
    Contrib. to the 11th Neutron and Ion Dosimetry Symposium (NEUDOS-11), iThemba LABS, Cape Town, South Africa, 12 - 16 October 2009.
  3. D.S. Sivia, J. Skilling:
    Data Analysis - A Bayesian Tutorial
    second ed. Oxford University Press, Oxford (2006).
  4. M. Reginatto:
    Bayesian approach for quantifying the uncertainty of neutron doses derived from spectrometric measurements
    Radiat. Prot. Dosim. 120, 64-69 (2006).
  5. M. Reginatto, P. Goldhagen, and S. Neumann:
    Spectrum unfolding, sensitivity analysis and propagation of uncertainties with the maximum entropy deconvolution code MAXED
    Nucl. Instr. And Meth. A476, 242-246 (2002).
  6. ICRP, International Commission on Radiological Protection, 1997. Conversion coefficients for use in radiological protection against external radiation. ICRP Publication 74. Annals of the ICRP Volume 26/3, Pergamon Press, Oxford.