Transcript Current pulse in the tungsten wire

A few thoughts on gamma densitometry of powder jet
Goran Skoro
27 October 2008
Density of tungsten powder jet?
Is it possible to map
the jet density profile?
Can the standard*
gamma spectrometry
equipment be used for
this purpose?
DET
g
*Here:
‘High’ activity gamma
source (75-Se, 192-Ir,
137-Cs);
Detector: NaI(Tl) or BGO;
MCA with possibility to
work in multiscaler regime
with ms dwell time.
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Gamma densitometry
Idea: If the jet diameter (size) is known, collimated
gamma radiation can be precisely pointed…
Jet cross-section
d
g
…and the difference in detector counting rate
with/without jet will give us the density, because:
N  N 0e

 N Av d
A
where N (N0) is the counting rate* with (without) jet,
 is the density, NAv is Avogadro number, A is atomic
mass of tungsten,  is (known) cross-section for
gamma interaction with tungsten atom, d is shown
above.
DET
The jet velocity is about 10
m/s, so it would be nice to have
the counting time at the level of
1 ms –> the density map
resolution will be about 1 cm.
Because of this we need a
fast Multiscaler.
*This is the counting rate of the spectral line which corresponds to the characteristic gamma energy
–> the spectral analysis is needed before counting.
Gamma densitometry – gamma sources, detectors, geometry,…
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Possible gamma sources:
75-Se: T1/2 = 120 days, Eg = 265 keV, bf = 60%;
192-Ir: T1/2 = 74 days, Eg = 317 keV, bf = 81%;
137-Cs: T1/2 = 30 years, Eg = 661 keV, bf = 100%.
Possible detectors:
Possible geometry:
NaI(Tl), BGO,…
d = 2 cm; l = 10 cm;
‘spot size’ -> r = 1 mm
2x2” NaI(Tl) good enough
For comparison only!
d
Source
DET
l
What is the minimal gamma source activity?
Important: The difference N0-N must be big enough in order not to be interpreted as
a statistical fluctuation (proportional to N). On the other hand, source activity has to
be as low as possible (for obvious reasons). This means that sensitivity of the density
measurements will have the lower limit.
What if this minimal value of ‘observable’ density is 10% of W density (and d = 1 cm)?
Results -> next page
Gamma densitometry – an example
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Statistical significance = (N0 -N)/3
 = 10% of W density, d = 1 cm
Source activity [Ci]
If > 1, the difference in counting rate is a result of absorption of gammas in tungsten jet,
not a statistical fluctuation. For example, we need a 137-Cs source with activity of > 1 Ci
to measure the density of 10% of W density. In the case of 192-Ir or 75-Se it is enough
to have ~ 100 mCi source.
Gamma densitometry – which gamma source?
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137-Cs: No! (of course);
high activity needed, long half-life, high (~500 kHz) counting rate*
75-Se: OK!
Counting rate below 50 kHz for 100 mCi source
192-Ir: OK!
Counting rate below 50 kHz for 100 mCi source
192-Ir preferable because of the shorter half-life and the price! **
£1100 for 20 Ci 192-Ir, £2000 for 20 Ci 75-Se.
We don’t need such a strong source (it seems that the upper limit in this case is
around 1 Ci*) so maybe a borrowing is a good idea…
To conclude: In general, this approach to measure the density looks possible. More
detailed optimisation needed…
*Another limiting factor (from the detector point of view). NaI(Tl) starts to lack in the performance at counting rates
between 200-500 kHz (dead time, pile-up, etc. corrections have to be performed).
** www.ndtequipment.co.uk, www.ndt-es.co.uk
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Update I
11 January 2009
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Gamma densitometer – a scheme
Shielding & collimator
Jet section
Source
Detector
Signal
processing
system
Detector and signal processing system
High voltage
NaI
Scintillator
SCA
PM tube
Preamplifier
Multiscaler
Amplifier
MCA
Multichannel analyser (MCA) can be used to measure a counting rate of corresponding photopeak or, if
we decide to use Single channel analyser (SCA) + Multiscaler for this, to monitor the gamma spectrum
(to control the possible drift of the amplifier gain, to see the effect of the pile-up, etc…)