Neutron source strength of a Th-228 gamma testing source for the BOREXINO experiment

For the BOREXINO experiment [1] in the Laboratori Nazionali del Gran Sasso (LNGS), a ²²⁸Th testing source for gamma radiation is required which does not emit more than 10 neutrons per second. The Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg and other European institutes have developed a 5.4 MBq ²²⁸Th radionuclide source with this secondary condition and manufactured a prototype [2]. Within the scope of a scientific cooperation, PTB’s Working Group "Neutron Spectrometry and Neutron Sources" has measured the neutron source strength of this source to check whether it can be used at the LNGS.

The challenge which lies in the measurement of the neutron source strength of the ²²⁸Th source was the expected low ratio of neutrons to gamma radiation of approx. 10-6. With a proportional counter filled with ³He gas (type SP9) of the company Centronic Ltd., UK, which is routinely used in the very well characterized neutron spectrometer NEMUS of PTB, the measurement signals caused by neutrons can be separated from those caused by gamma radiation and by other interfering signals. To check the adjusted thresholds in the pulse height spectrum for the separation of the two radiation types under the condition of the very high gamma fraction, the SP9 counter was irradiated in our source irradiation facility separately with neutrons and gamma radiation and subsequently with a combination of the two radiation types. With these measurements, it was ensured in addition that the dead time of the measuring system and the detection probability for neutrons are not changed by additional gamma radiation.

For a ThO₂ solution - the basis of the thorium testing source - Monte Carlo simulation calculations yield a mean neutron energy of 2.41 MeV. As the ³He counter is sensitive only to thermal neutrons (up to a few eV), a measuring device was used which is routinely used for the quality control of the ³He counters. Here, the ³He counter and a source are arranged in a paraffin block as shown in Figure 1.

Figure 1 : ³He counter (type SP9) and neutron source in the paraffin block, dimensions in mm

The predicted low neutron source strength of the ²²⁸Th source required a measuring time of several days. As the measurements were performed in a laboratory building on the premises of PTB - and not in an underground measurement laboratory - the counter events to be assigned to neutrons from cosmic radiation are to be subtracted as background. Here, the influence of air pressure - which becomes evident by an anticorrelation between the air pressure and the neutron fluence rate - is to be taken into account [3]. In a measurement series of 149 measurements, with a measurement time of 4 hours each, a mean count rate of neutrons from cosmic radiation of  was determined as background (bg). For the measurements without the Th source (orange symbols), Figure 2 shows that the anticorrelation cannot be observed due to statistical uncertainties which are too large and can be neglected within the scope of the uncertainties.

Figure 2 : Number of counts in the 3He counter for a measurement interval of 4 hours (ordinate on the left) and air pressure measured in the period from 29-03-2010 to 04-05-2010 (ordinate on the right)

After the ²²⁸Th source had been inserted, 66 measurements, with a measuring time of 4 hours each, were performed from which a mean count rate of  was determined. These counting events (see Fig. 2, red symbols) originate from the combination of neutrons from cosmic radiation and neutrons from the ²²⁸Th source (bgTh). For the determination of the absolute neutron source strength, the conversion factor which links the counting events with the neutron source strength, must be determined. Investigations have shown that a ²⁴¹AmB source with a known neutron source strength is suited best for the determination of the conversion factor.

Taking account of the measured background - and by applying the conversion factor - a neutron source strength of the ²²⁸Th source prepared by the MPIK for the Borexino experiment of  was determined (reference date: 29-04-2010) so that the above-mentioned condition for being used at the LNGS has been fulfilled.

Literature

1. borex.lngs.infn.it
2. R. Dressler et al.:
   Production and characterization of a custom-made ²²⁸Th source with reduced neutron flux for the BOREXINO experiment.
   To be subm. to Nucl. Instrum. Meth. A.
3. B. Wiegel, A.V. Alevra, M. Matzke, U. J. Schrewe and J. Wittstock:
   Spectrometry with the PTB Neutron Multisphere Spectrometer (NEMUS) at Flight Altitudes and at Ground Level,
   Nucl. Instrum. Meth. A476, 52-57 (2002).