The n_TOF collaboration CERN - The European Organization for Nuclear Research

The Total Absorption Calorimeter (TAC)

The Total Absorption Calorimeter (TAC) is a 4pi segmented array made of 42 BaF₂ crystals of two different shapes (pentagonal and hexagonal) specially built for detecting the g-ray cascades emitted in neutron capture reactions. Full details on the characteristics and performance of the detector are given in Ref. [1].

The 40 crystals, encapsulated in ¹⁰B-loaded carbon fiber, form a spherical shell of 15 cm thickness with an inner diameter of 20 cm. The TAC shows a nearly 100% efficiency for detecting capture cascades, providing an energy resolution of 16% at 662 keV and 6% at 6.1 MeV. A 5 cm thick spherical shell made of C₁₂H₂₀O₂⁶Li₂ is placed in the inner hole of the TAC for moderating and partially absorbing the neutrons that are scattered in the sample. The combination of this moderator with the ¹⁰B-loaded carbon fiber capsules results in a neutron sensitivity (i.e. efficiency for detecting neutrons) lower than 1% in the neutron energy range of interest.

The characteristics of the TAC provide the means for performing high quality neutron capture measurements of small mass and/or radioactive samples. For instance, the high efficiency of the TAC and the high intensity of the n_TOF neutron flux allow reducing the background caused by the intrinsic activity in the case of radioactive samples. Furthermore, the high total absorption efficiency, segmentation and energy resolution of the detector allow discriminating between different reactions attending to their Q-values and g-ray multiplicities. This allows one to improve further the capture over background ratio.

The TAC has been used successfully during the 2004 n_TOF campaign to measured the neutron capture cross sections of ²³³U [1],  ²³⁴U [2],²³⁷Np [3], ²⁴⁰Pu [3] and ²⁴³Am. In addition, the n_TOF-Ph2 experimental program for 2010 and 2011 includes measuring the capture cross sections of ²³⁸U, ²⁴¹Am and ²⁴³Am, a proposal [5] that has been already approved by the CERN INTC.

References:
[1] C. Guerrero et al., The n_TOF Total Absorption Calorimeter for neutron capture measurements at CERN, Nucl. Int. and Meth. A 608 (2009) 424-433
[2] E. Berthoumieux et al., Simultaneous measurement of the neutron capture and fission yields of ²³³U, Int. Conf. Nuclear Data for Sci. & Tech. (ND2007), Nice-France, April 2007.
[3] W. Dridi et al., Measurement of the neutron capture cross section of ²³⁴U in n_TOF at CERN, Topical Meeting on Reactor Physics – PHYSOR 2006, Vancouver, Canada, September 2006. ANS-ISBN: 0-89448-697-7. PHYSOR2006 – C032.
[4] C. Guerrero et al., The neutron capture cross sections of ²³⁷Np(n,g) and ²⁴⁰Pu(n,g) and its relevance in the transmutation of nuclear waste, Int. Conf. Nuclear Data for Sci. & Tech. (ND2007), Nice-France, April 2007.
[5] D. Cano-Ott and F. Gunsing et al., Neutron capture cross section measurements of ²³⁸U, ²⁴¹Am and ²⁴³Am at n_TOF, CERN/INTC/NTOF15 (2009)