A few thoughts on gamma densitometry of powder jet

Goran Skoro

27 October 2008

Density of tungsten powder jet?

DET

Is it possible to map the jet density profile?

Can the standard* gamma spectrometry equipment be used for this purpose?

*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.

2

Gamma densitometry

Idea: If the jet diameter (size) is known, collimated gamma radiation can be precisely pointed…

…and the difference in detector counting rate with/without jet will give us the density, because:

dNA Av

eNN

0

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

dJet cross-section

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.

*This is the counting rate of the spectral line which corresponds to the characteristic gamma energy –> the spectral analysis is needed before counting.

Because of this we need a fast Multiscaler.

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Gamma densitometry – gamma sources, detectors, geometry,…

Possible gamma sources:

75-Se: T1/2 = 120 days, E = 265 keV, bf = 60%;

192-Ir: T1/2 = 74 days, E = 317 keV, bf = 81%;

137-Cs: T1/2 = 30 years, E = 661 keV, bf = 100%.

For comparison only!

Possible detectors:

NaI(Tl), BGO,…

2x2” NaI(Tl) good enough

DET

d

Source

l

Possible geometry:

d = 2 cm; l = 10 cm;‘spot size’ -> r = 1 mm

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

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Gamma densitometry – an exampleS

tati

sti

cal sig

nifi

can

ce =

(N

0 -

N)/

3

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.

= 10% of W density, d = 1 cm

5

Gamma densitometry – which gamma source?

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

*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).

192-Ir preferable because of the shorter half-life and the price! **

£1100 for 20 Ci 192-Ir, £2000 for 20 Ci 75-Se.

** www.ndtequipment.co.uk, www.ndt-es.co.uk

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…

6

Update I

11 January 2009

7

Gamma densitometer – a scheme8

Source

Shielding & collimator Jet section Detector

Signal processing

system

Detector and signal processing system

NaI Scintillator PM tube Preamplifie

r Amplifier

MCA

SCA Multiscaler

High voltage

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…)