DÖME Low-Level Gamma-Spectroscopy Facility

Instrument responsible: Zoltán Kis
Contact: dome@bnc.hu

The increasing need for low-background counting led to the installation of a permanent counting station in the cold-neutron guide hall of Budapest Neutron Centre in 2010. The purpose of DÖME (DÖgnehéz Mérő Eszköz or “damn heavy measuring tool”) is to assist the in-beam activation measurements with off-line counting option on a routine basis and to allow the measurement of environmental samples of low activities. The facility’s main features are instant and local availability and highly accurate activity measurement.

The low-level counting station consists of an iron chamber manufactured from pre-World War II iron (recycled material of the bombed Elisabeth-bridge, 1945), and is, therefore, free of any man-made radioactivity. The wall of the chamber is 155 mm thick and has a graded shielding (148.5 mm Fe, 5 mm Pb, and 1.5 mm Cu layers). The chamber with an internal dimension of 800×800×800 mm³ accommodates a Canberra GR1319 HPGe detector of 13% relative efficiency with a BigMAC cryostat along its horizontal diagonal. This geometry allows sample-to-detector distances up to 250 mm.

As an option for targeted measurements of low-energy lines, a Canberra Low Energy HPGe (GL1018) detector is also available with better energy resolution but for a narrower energy range (up to about 1 MeV). A Canberra DSA-2000 digital gamma spectrometer with 16k channels is used for the data collection. With the GR1319 detector, the facility has a spectral background of about 1.39 cps over the energy range of 7-3150 keV, which compares to 211 cps outside the chamber (i.e. a reduction factor of 152). Dead-time losses during the experiments are typically below 1%.

In-beam-activated powder samples can be measured in the Teflon bag used for the PGAA or NIPS-NORMA experiment. Solid samples can be analyzed as received. For powdered environmental samples an easily re-sealable, radon-gastight sample holder from high-density polyethylene was fabricated.

The three main parameters of the facility are detection efficiency, channel-to-energy calibration with nonlinearity, and energy resolution. These are established using commercially available certified radioactive sources (¹⁵²Eu, ²⁰⁷Bi, ⁶⁰Co, ¹³³Ba, ²²⁶Ra). The system is calibrated for contact geometry as well as for 32, 167, and 237 mm distances. The Efficiency, Nonlinearity, and FWHM Analysis modules of the Hypermet-PC software are used for such calculations, while for the efficiency determination of larger, close-to-detector samples the Excel-based Efftran efficiency-transfer program and its macros are used.

Figure. 1: Sample geometries at the DÖME station
  Left: off-line counting of in-beam activated samples. Right: HDPE container for environmental samples

                         Figure 2. The low-level counting chamber DÖME

     Figure 3: The absolute full-energy peak efficiency for point sources
                          at 167 mm in the DÖME counting station.

                              Figure 4: Nonlinearity and energy resolution of DÖME counting station

References:

Z Kis, P Völgyesi, Z Szabó. (2013) DÖME: Revitalizing a low-background counting chamber and developing a radon-tight sample holder for gamma-ray spectroscopy measurements. J Radioanal Nucl Chem. 298(3) 2029-35. DOI: 10.1007/s10967-013-2691-8

Szentmiklósi L, Maróti B, Párkányi D, Harsányi I, Révay Z  (2018): High-energy detector calibration data for k0-neutron activation analysis. J Radioanal Nucl Chem. 315(3) 743–750