Neutron-Induced Partial
Cross-Section Measurements on
76Ge Motivated by the Majorana
Experiment
76Georana
Decay Search
ResearchProject
Experiences0
For Undergraduates
(REU)
Stephen Hilderbrand
Neutron-Induced Gamma-Ray Cross-Section Measurements
on 76Ge Motivated By the Majorana Experiment
The neutrinos, under the three types e, , and  (electron, muon, and tau neutrinos)
have been mysterious since some experimental evidence surfaced in 1956 pointing to
their existence. We know four fundamental interactions between the particles: the strong
interaction, the electromagnetic interaction, the weak interaction and the gravitational
interaction. The neutrinos are only concerned with the weak interaction and this allows
them to pass through the earth. They interact possibly one time out of one billion in the
huge apparatuses built to detect them. The neutrino also seems to have a remarkable
property: its spin is always oriented in the direction opposite to its velocity (it is said to
be of left helicity). The anti-neutrino is always of right helicity (where the spin is in the
same direction as the velocity). The standard model declares that the neutrino has a zero
mass, a zero charge and a spin 1/2. However experimental evidence in the past 10-12
years suggests that neutrinos in fact are massive particles. The elusive neutrino’s very
small mass is now estimated to be only 1/500,000th that of an electron.
The principal goal of the Majorana Collaboration is to study neutrinoless double beta
decay (0) in order to verify that the neutrino is its own anti-particle; and if so, what is
the mass of the neutrino? If Majorana is right (that is that the neutrino is identical to the
anti-neutrino) or if the neutrino is massive, then the neutrino will not always respect the
following observed rule: a neutrino is always left and an anti-neutrino is always right.
The only present experimental method capable of attempting to answer both of these
questions is the search for the neutrinoless double beta decay. If such a rare process were
to occur in 76Ge(as it is thought to), the maximum energy that the two emitted electrons
can deposit in a detector is its 2039 keV endpoint energy. (See Fig.1) Observation of a
sharp peak at the  endpoint will confirm 0 as a decay mode, and determination of
the partial width will determine the matrix element which depends directly on the 76Ge
electron neutrino mass.
(Fig.1) 2 and 0 energies
Double-Beta Decay (ββ)
e-
(neutrinos emitted)
e
Z
e
Z+1
eZ+2
(Fig.2)
Neutrinoless (0νββ)
Double-Beta Decay
(no neutrinos emitted)
e-
p+
e-
p+
e
n
n
(Fig.3)
In standard double beta decay (Fig.2): As the nucleus decays into a new nucleus,
two neutrons convert to protons, releasing two electrons and two antineutrinos. In
neutrinoless double beta decay (Fig.3): If neutrinos and antineutrinos are Majorana, an
antineutrino can be emitted from the first neutron then absorbed as a neutrino by the
second. Only the two electrons will be released, carrying all the decay energy.
In other words with 2 the electrons and neutrinos share the energy. However in 0
a spike occurs at 2039kev. This is a result of neutrino annihilation where the electrons
carry all of the decay energy.
Has 0 been observed? The Heidelberg-Moscow experiment1 claimed to have
observed 0vββ peak around 2040 keV in 76Ge. Importantly though this claim is yet to be
verified. See (Fig. 4). The Majorana collaboration seeks to verify this claim.
2
bWn y
With
2
bWn y
With
(Fig.4)
1H.V. Klapdor-Kleingrothaus et al.,
Phys. Lett. B 586 (2004) 198
The proposed Majorana setup (See Fig.5 below) requires a thorough understanding of
background sources.
(Fig.5) The Proposed Majorana Setup
The Majorana Project will operate with High Purity Germanium (HPGe) detectors
located deep underground to achieve a low background environment. The simplest and
most reliable way to achieve lower cosmic-ray induced backgrounds is to do experiments
at greater depths. Recent advances in signal processing and detector design have enabled
scientists to further understand these background sources(such as cosmic rays).
There had been considerable interest in developing a deep underground laboratory in
the United States to carry out a diverse program of scientific research, much of it directly
related to particle physics. Recently (in summer of 2007) the National Science
Foundation chose South Dakota's closed Homestake Gold Mine as the site for a new
underground physics lab. It is called the Deep Underground Science and Engineering
Laboratory(DUSL). Homestake (at 8,000 feet) is deep enough to provide the very low
background required for a variety of science experiments. In fact the lab will be the
largest and deepest facility of its kind in the world. Homestake will be a dedicated facility
for science only, with no conflicts with other users. Majorana may have a home there.
Our experiment and TUNL’s contribution
This next generation of the  decay experiment will search for the 76Ge  decay.
Thus, it is important to understand how levels in 76Ge are populated. The key to 0
detection lies in the ability to reduce intrinsic and extrinsic radioactive backgrounds.
Measurements of neutron excitation in lead, copper, and germanium in search of specific
excited state decays contribute to the understanding neutron-induced backgrounds.
Understanding these excitations of the shielding and detector materials for neutrinoless
double-beta decay experiments are crucial for interpreting results and establishing
shielding requirements. Our experiment sought to measure neutron induced partial cross
sections from (n, xn’) with a 86% enriched 76Ge target. We put pulsed mono-energetic
neutrons incident on an enriched germanium76 target(10.84g). Our target was placed
between two 0.1mm Iron(Fe) foils for normalization purposes. Efficiency and energy
calibrations were done with 56,60Co, 137Cs, 152Eu, 54Mn, 22Na: sources of known
intensity. We used 4 HPGE detectors; 3 clovers and one planar detector. The three
segmented HPGe Clover detectors were positioned at 90, 245, and 315◦ relative to the
neutron beam axis. Here these detectors were used to measure the angular distribution of
the emitted gamma-rays. The neutron beam created by the reaction in the gas cell passes
through the wall in a collimated channel. The collimator has inserts that can be
exchanged to provide collimated beams of different dimensions. Our beam was 5cm in
diameter. The spectrum was obtained over a course of three days. The neutron induced
cross sections of the strongest transitions in 76Ge and 74Ge at 8 and 12 MeV will be
evaluated from the current data. If the 2039 keV line from the beta decay of 76Ge were to
exist, this line would be indistinguishable from the background. This is why it is
important to know partial cross sections of these neutron-induced 76Ge(n,n’g)76Ge to
allow us to subtract these events out of the 0vBB decay spectrum.
Preliminary Results
(Fig.6)
Shown in Fig.6 is the gamma-ray spectrum from one crystal at En = 12 MeV for the
region of interest. This is a preliminary gamma-ray spectrum from the interaction of 12
MeV neutrons with 86% enriched 76Ge. The dotted histogram is the beam correlated
spectrum and the solid histogram is the uncorrelated background.
564 keV/76Ge
546 keV/Ge76
76
431 keV/ Ge
595 keV/ 74Ge
847 keV/ 56Fe,
76
Ge
(Fig.7) En = 12 MeV Sample Spectra; Ge Peaks of Interest
References
1. J.E. Ellis. PNNL Majorana Information Resource Page. <http://majorana.pnl.gov/>.
7 July 2006.
2. D. Mei. Backgrounds in the Next Generation Double-Beta Decay Experiments. Los
Alamos National Lab.
.