Implementation and Utilization of Positron Annihilation Spectrometry Methods at the PULSTAR ...

17 Pages·2014·1.68 MB·English

by  Jesse Holmes

Joint IGORR 2014 & IAEA Technical Meeting

November 17 – 21, 2014 • Bariloche, Argentina

Implementation and Utilization of
Positron Annihilation Spectrometry Methods
at the
PULSTAR Reactor

A. I. Hawari, M. Liu

Nuclear Reactor Program
Department of Nuclear Engineering
North Carolina State University
Raleigh, North Carolina, USA
Motivation
 Positrons can be a useful tool for the
nondestructive characterization of nanometer
scale properties of materials

 Understanding and controlling nanometer-sized
pores is becoming increasingly important to the
field of nanotechnology
 Hydrogen storage, interlayer dielectrics in
microchips and polymer electrolytic membranes in
fuel cells, etc.
Positron Sources
 Isotopic sources – typical
 For example, Na-22 t ≈ 2.6 years
1/2
 Intense sources (106 e+/s) are expensive and limited in supply
 Lose intensity with time (source decay)
 Always on (radiation protection concern)

 Reactor based sources
 At least 2 orders of magnitude more intense than isotopic
sources (108 e+/s)
 On-demand beams
 On a university campus, can be highly accessible and relatively
inexpensive to use

 Accelerator based sources
 Similar attributes to reactor based sources
 However, need powerful accelerators – Very Expensive
machines
 In the US, these existed on national lab sites
PULSTAR Reactor
 1-MW power
 Open tank
 Light water moderated and
cooled
 5 x 5 array of fuel
assemblies
 5 x 5 array of pins
 Sintered UO pellets
2
 4% enriched
 Major Capabilities

 Neutron powder
diffraction

 Neutron imaging

 Intense positron beam

 Ultracold neutron source
(under testing)

 Neutron activation
analysis

 In-pool irradiation testing
facilities

PULSTAR reactor bay
Intense Positron Beam Facility
 A concrete shielding cave surrounds the beam and switchyard
 Beam can be steered to two spectrometers
 A new positron generator and a third spectrometer with micron-sized beam
spot (microprobe) is under development
Bulk Positron System
 Two 1”x1” plastic
High Voltage Sample High Voltage
scintillators with
Power Supply Scintillator Power Supply
Hamamatsu H3378-50
1.27MeV γ 511keV γ
PMT PMT photomultiplier tubes
START STOP
 LeCroy 7300A digital
22Na source sealed
Hamamatsu Hamamatsu
by 7.6μm Kapton films oscilloscope to
1949-51 1949-51
Time to Analog to Digital
digitize the raw PMT
Amplitude Convertor and Multi-
Convertor Channel Analyzer pulses and acquire
Data Analysis
Analog System PALS spectrum
Delay
tilp Coincident Trigger giS  Time resolution is
S CFD SCA SCA CFD n
la (Ortec 583B) SCA Voltage Sum (Ortec 583B) la ~200 ps
n S
g p
iS til  Analog system

LeCroy wavepro
operating in parallel,
7300A Digital
Oscilloscope
and is used for count
rate correction and
calibration
Positron Annihilation Spectrometry
The probe - positron (e+)
 Antimatter of electron
 Naturally diffuse to vacancies and voids
 Annihilation gammas related to materials free volume
and vacancy properties
Positron Annihilation Lifetime Spectroscopy
(PALS) – measures the decay lifetime of positron in materials which is
directly related to the extent, size, and/or type of vacancies, free volume
voids, and engineered nano-pores
Doppler Broadening Spectroscopy (DBS) –
measures
the Doppler effect of the annihilation 511keV gammas, which is directly related
to the momentum of the electron-positron pair
parallel to the gammas
Positron User Facility for
Materials Research
Polyamide Active Layer
W
a
Polysulfone
t
e
r
Polyester
f
l
o
w

Understanding and controlling nanometer-sized pores is becoming
increasingly important to the field of nanotechnology
 Metal and semiconductor (defects, fatigue)
 Graphite (radiation damage)
 Polymers thin films, nanocomposite, morphology
 Diffusion barriers, capping layers, pore sealing
 Membranes for gas/liquid filtration, etc.
 Nanoporous low-k dielectrics
 High surface area materials (energy storage, CO sequestration)
2
Organic photovoltaic bulk heterojunction
thin film (with NIST)
 low-cost, large-area fabrication (printing, doctor-blading,
painting), light weight, flexible/foldable/rollable substrates
 Challenges: low efficiencies related to uncontrolled and poorly
understood interface composition and active layer morphology