JAEA-Conf 2011-002 - 日本原子力研究開発機構

layout of the RPI LINAC facility.
JAEA-Conf 2011-002
The RPI capture detector 9 is a 30.5-cm-diam30.5-cm-high hollow cylinder that contains 16 sections
of thallium activated sodium iodide NaI(Tl) scintillator crystals. The cylinder is split perpendicular to its axis
into two rings, and each ring contains eight equally sized pie-shaped NaI(Tl) segments. Each NaI(Tl) segment is
optically isolated and hermetically sealed within an aluminum can and mounted on an RCA 8575 photomultiplier.
The samples are placed by a computer-controlled sample changer into the center of the capture detector. The
sample changer is capable of holding up to eight samples. The boron carbide liner (enriched in 10 B) around the
sample reduces the scattering background by absorbing the neutrons that are scattered from the sample and
preventing the scattered neutrons from being detected in the NaI(Tl) crystal. The detector system discriminates
against the 478-keV gamma-ray from 10 B (n;,) reactions. The efficiency of the capture detector is ~75 % for a
single 2 MeV gamma-ray. The efficiency of a detecting a capture event in gadolinium is close to 100 % and
varies slightly by isotope and sample thickness. Reference 10 contains a description of the detector and its
signal-processing electronics. Neutron capture data-taking and data-reduction techniques at the RPI are
described in Refs. 11 and 12.
The large amount of data collected in each capture measurement was subject to statistical integrity
checks to verify the stability of the linac, the capture detector, and associated beam monitors. Any data that failed
the integrity test were eliminated. Next, the data were dead-time corrected, normalized to beam monitors, and
summed. The background was determined using normalized data measured with an empty aluminum can
mounted on the sample changer. This background was subtracted from the normalized and summed capture
spectra. The 16 individual capture spectrums were then summed into a single total spectrum.
When an incident neutron is captured in the sample, a compound nucleus in an excited state is formed.
The compound nucleus then de-excites to the ground state with the subsequent emission of gamma rays. The
detection of these gamma rays allows one to measure the fraction of neutrons of a given energy that are captured
if the incident neutron flux is known. This fraction of captures is known as the capture yield. Thus, for a uniform
thickness sample and a parallel neutron beam incident perpendicularly to this sample, the capture yield is defined
as the number of detected capture gamma rays divided by the product of the detector efficiency times the number
of incident neutrons. Mathematically speaking, the capture yield is defined as the number of captures per
incident neutron. In time of flight measurements the capture yield, Yi in TOF channel i was calculated by
Y
C B
i i
i
Ksmi
where
Ci= dead-time-corrected and monitor-normalized count rate of the sample measurement
Bi= dead-time-corrected and monitor-normalized background counting rate
K= detector efficiency and flux normalization factor
smi= smoothed, background-subtracted, and monitor-normalized neutron flux shape.
The incident neutron flux shape was determined with the use of a thick 10 B4C sample that is mounted on the