Instrumentation & Methods:
Laser Phosphorimetry, Uranium
Richard Sheibley
Pennsylvania Dept of Env
Protection
Laser Phosphorimeter
{
{
{
UV excitation by pulsed
nitrogen laser 337nm
Green luminescence at 494,
516 and 540
Excitation 3-4 X 10-9sec
Laser Phosphorimeter
{
{
Measure luminescence when
laser is off
Use method of standard
addition
1
Instrumentation & Methods:
Alpha Spectroscopy, Uranium
Lynn West
Wisconsin State Lab of Hygiene
Review of Radioactive Modes of Decay
{
Properties of Alpha Decay
z
z
z
Progeny loses of 4 AMU.
Progeny loses 2 nuclear charges
Often followed by emission of gamma
226
88 Ra
222
Rn
86
+ 42He + energy
Review of Radioactive Modes of
Decay, Cont.
Properties of
Alpha Decay
z
z
Alpha particle and
progeny (recoil
nucleus) have welldefined energies
spectroscopy based
on alpha-particle
energies is possible
Counts
{
4.5
5.5
Energy (MeV)
Alpha spectrum at the theoretical
limit of energy resolution
2
Instrumentation –
Alpha Spectroscopy
{
{
{
{
{
{
Types of detectors
Resolution
Spectroscopy
Calibration/Efficiency
Sample Preparation
Daily Instrument Checks
Types of detectors (Alpha
Spectroscopy)
{
Older technology
z
z
z
z
{
Diffused junction detector (DJD)
Surface barrier silicon detectors (SSB)
Ion Implanted Layers
Fully depleted detectors
State-of-the-art technology
z
Passivated implanted planar silicon
detector (PIPS)
PIPS
{
{
{
{
{
Good alpha resolution due very thin
uniform entrance window
Surface is more rugged and can be
cleaned
Low leakage current
Low noise
Bakable at high temperatures
3
Alpha Spectrometer Detector
{
{
{
An example of a
passivated
implanted planar
silicon detector
600 mm2 active
area
Resolution of 24
keV (FWHM)
Alpha Spectrometer
Resolution
{
{
{
{
Broadening of peaks is due to various
sources of leakage current – “Noise”
Low energy tails result from trapping of
charge carriers which results from the
incomplete collection of the total energy
deposited
Good resolution increases sensitivity
(background below peak is reduced)
Resolution of 10 keV is achievable with
PIPS (controlled conditions)
4
Typical Alpha Spectrum
Calibration/Efficiency
{
{
{
{
Energy calibration
Efficiency can be determined
mathematically using Monte-Carlo
simulation
Efficiency can be determined using
a NIST traceable standard in same
geometry as samples
Efficiency determination not always
needed with tracers
Sample Preparation
{
{
Final sample must be very thin to
insure high resolution and minimize
tailing. Also should stable & rugged
The following mounting techniques
are commonly used:
z
z
z
Electrodeposition
Micro precipitation
Evaporation from organic solutions
{
Organics must be completely removed
5
Sample Preparation
{
Chemical and radiochemical interferences
must be removed during preparation
z
z
Nuclides must be removed which have
energies close to the energies of the nuclide of
interest, ie 15 to 30 keV
{ Ion exchange
{ Precipitation/coprecipitation techniques
{ Chemical extractions
Chemicals which might damage detector must
be elimanted
Sample Preparation
{
{
A radioactive tracer is used to
determine the recovery of the
nuclide of interest
Since a tracer is added to every
sample, a matrix spiked sample is
not required
Sample Counting
{
{
Mounts with a
small negative
voltage can be
used to help
attract the recoil
nucleus away from
the detector
Reduces detector
contamination
6
Sample Counting
{
{
{
Analyst can choice
distance from
detector
Trade off is
between efficiency
& resolution
Count performed
slightly above
atm. pressure to
reduce
contamination
Daily instrument checks
{
{
One hour background
Pulser check
z
Stability check
7