Solvent heat up due to gamma ray

[raj26]

I am planning to use a gamma ray densitometer to measure the local concentration of slurry flow in a pipe loop.

Since i am using an organic solvent(flammable) to make the slurry, i am worried that gamma ray passing through it will heat it up significantly and cause hazard. I would like to know the possible temperature rise in the solvent if the gamma ray passes through it for duration of 10 minutes.

(I am predicting a possible rise of 3-4 degree celsius due to gamma ray absorption and flow friction)

Thanks
Rajesh

 

[Bob S]

What is the slurry composition, and what is the maximum diameter of the pipe (what pipe material?) that the slurry is in? Do you just need density, or also flow rate (hard to do)? Do you need or want to use a high-voltage x-ray source, or a radioactive source? Is pipe round? Do you want to measure the total slurry, or selective composition? (HV x-rays are better for selective composition). What accuracy (or precision) is needed? What spatial resolution? What time resolution? Are you thinking of a total count rate in a detector like sodium iodide (sodium iodide has higher sensitivity than solid state detector)?

I don't think there is any worry from gamma-ray heating, even if radiation is continuous, but the radiation may be several rads per hour, depending on answers to above questions.

[added] Miscellaneous conversion factors
1 rad/hour = 100 ergs per gram-hour = 10^-5 joules per gram-hour = 3 nanowatts per gram.

1 million 100-KeV photons absorbed per gram per second is 16 nanowatts per gram.

The absorption coefficient for 100 KeV x –rays is 0.15 cm² per gram = 0.15 cm^-1 in water.

Bob S

 

[raj26]

First of all, thanks for the response.

I am planning to test coke+MTBE slurry in 52.5mm ID plexiglass tube(thickness of 1mm). I will vary the the concentration from 15-35%. The tube is cylindrcial.

The literature is loaded with references of using gamma ray for density measuremnts in slurry flow. So i felt more comfortable in using it as compared to X-ray. Also x-ray is impermeable through certain materials.

i would like to have highest accuracy possible( any precaution for that)

I will be be using sodium iodide detector with count rate.

Finally , i don't want any significant temperature rise and avoid any flammability issue.

 

[Bob S]

How does slurry density change with coke concentration? Is the coke a dry powder byproduct of coal? What is density range of slurry? Any significant change in concentration of constituents with concentration of coke? What radioactive source do you want to use? Cs-137 (661 KeV)? Gamma attenuation coeff in water = ~0.1 cm²/gram, so 10 cm is ~1/e attenuation. How do you propose doing NaI count rate calibration? Pure water, hydrocarbon, or pure MTBE? How long is density measurement period? 1 second, 10 seconds, 1 minute (This determines radioactive source strength)? Do you have radiatian safety limits on radioactive source strength (milliCuries) or personnel exposure (rads/hour)?

Bob S

 

[raj26]

Dry coke particles of size 200-600 micrometers will be used to make the slurry. Slurry conc will be varied by manual loading. The density range of slurry will be 800-1400kg/m3

I will use Cs-137. NAI count rate calibration is done both with water and pure MTBE, as both of their slurries are tested.

I will keep a peroid of 1 minute for accuracy, but i don't know how this affects the strength.

I haven't thought about safety limits

 

[Bob S]

Here are some numbers.

Let's start with a 1 milliCurie Cs-137 source: 3.7 x 10⁷ decays per second

About 85% of the decays produce a 661 KeV gamma, so 3.1 x 10⁷ gammas per second.

If the source is 150 mm away from a 3"x3" NaI, with a 20 mm by 50 mm lead aperture in front, the solid angle is 20 x 50/(4π x 150²) = 3.5 x 10^-3. This is to ensure that the NaI looks only at the thickest part of the pipe.

So the counting rate without any fluid in the tube is about 108,000 counts per second.

Very roughly the attenuation constant is 0.08 cm² per gram.This is very similar for water, hydrocarbons, and coke slurry (carbon).

For 5 cm of density 1 grams/cm³ fluid, the counting rate is

108,000 e^-μx = 67,000 counts per second

For density 1.4, the counting rate is about 57,000 counts per second.

So in 1 minute, the total difference in counts between 1 and 1.4 grams/cm³ is about 600,000 counts. This should give good precision on measuring density. You could reduce the source strength a factor of 10, but your density discrimination precision will be marginal.

The counting rates are not excessive (pileup). The aperture in front of the NaI can be adjusted to change the counting rate.

The total heating energy emitted by the source is ~3.7 x 10⁷ x 10⁶ eV x 1.6 x 10^-19 = 6 microwatts. No problem.

There should be lead shielding around this for personnel radiation protection. You may need to get a license for the source.

Bob S

 

[raj26]

Thanks for the numbers. You seem to be an expert in this field.

Btw, I read on the internet that only non-ionising rays are heat generating. Since gamma ray is ionising, it shouldn't heat up the test sample. How true is this argument.

Thanks
Rajesh

 

[Bob S]

Thanks for the numbers. You seem to be an expert in this field.

Btw, I read on the internet that only non-ionising rays are heat generating. Since gamma ray is ionising, it shouldn't heat up the test sample. How true is this argument.

The 661-KeV photons from Cs-137 decay collide with electrons in matter (called Compton scattering), and the recoil electrons ionize and heat up the matter. Eventually, all the recoil electron ionization leads to heat generation, in solid, liquid, or gaseous matter. The only energy that escapes comes from neutrinos (very small percentage).

In NaI(Tl), some of the the recoil electron energy is converted to visible light, which is detected by a photomultiplier tube.

Bob S

 

[raj26]

Hi Bob

I am ordering a gamma densitometer soon. Can you please tell me what all details should i give to the suppliers(one for the source, one for the detector) to get the right specification for my measurements.

Thanks
Rajesh

 

[Bob S]

The components you will need for your system are:
1) Cs-137 source, about 1 mCi
2) integrated 3" x 3" NaI(Tl) crystal with a photomultiplier tube (PMT)
3) High voltage supply for PMT
4) preamp (maybe)
5) Amplitude discriminator or pulse height analyzer
6) Automated readout system?

In order for you to get about ±0.01 resolution on the density (1 to 1.4 grams/cm³) in the slurry in the 52-mm diameter tube in 1 minute, you will need a source of approximately 1 milliCurie (mCi) strength (The exact number to be determined later). You should contact local authorities and determine whether you need a license or permit for this source (you will), and whether there are providers in your area.

You should determine what kind of readout you want. It could be as simple as an analog meter, or a counter/scaler (digital readout), or a pulse height analyzer (PHA). Is the readout going to be used in any automated feedback control loop, or manually read out?

Depending on the complexity of this system, you probably should have some local company assist you in the installation/integration of the system. In particlular, the Cs-137 source is a very strong source and is a health hazard. The electronics could be be non-standard, so a knowledgeable technician should work with you.

If there are limits in the strength of the Cs-137 source you can use, please let us know.

Bob S

 

[raj26]

Hi Bob

I am using a 100mm diameter tube .Does this change the source strength and other specifics. What aperture thickness do you recommend. Can you please specify the source strength required for my system in KeV

Regarding data acquisition, i just need a digital reading of the local density at different vertical distances from the bottom of the pipe. Can i also a get a digital reading of the height i am traversing from the bottom of the pipe.
These two data should suffice for me.

Thanks for all the information
Rajesh

 

[Bob S]

Hi Rajesh-

Here are some new numbers for a 10-cm diameter pipe.

Simple geometric solid angle for a 3" by 3" NaI(Tl) crystal at 20 cm (from source to center of crystal)

F = πr²/4πR² = 3.8²/4·20² = 9 x 10^-3

Cs-137 branching ratio to 661-keV gamma = 0.86
NaI detection efficiency = 0.9
Photopeak efficiency = 0.5

see http://www.google.com/url?sa=t&sour...0PjSCw&usg=AFQjCNFEEZdsPo_UitlzmZMXgv5_uv3Wvw

So counting rate in NaI with air for a 1 mCi Cs-137 source is

3.7 x 10⁷ x 0.86 x 9 x 10^-3 x 0.9 x 0.6 = 125,000 counts per second (cps)

The absorption coefficient for 661-keV gammas in water is 0.09 cm² per gram

so for 10 cm water, the absorption is e^{-0.09 x 10} = e^-0.9 = 0.406

So for 10 cm slurry with density 1.4, the absorption is e^{-0.09 x 10 x 1.4} = e^-1.26 = 0.284

So the counting rates are

air 125,000 cps
water 51.000 cps
slurry 36,000 cps

So for 1 second counting, the difference between slurry (density 1.4) and water, including counting uncertainty statistics, is (51,000 - 36,000) ± (230² + 190²)^½ = 15,000 ± 300 cps

this is sufficient to yield a slurry density measurement resolution of about

300 x (1.4 - 1.0)/15,000 = 0.008 grams per cm³

So if counting for 1 minute to get the above total counts is adequate, then the source could be ≈20 μCi. If we allow for a lead collimator aperture in front of the NaI, then 50 to 100 μCi would be adequate.

The lead aperture collimator would be ≈4 cm wide by 7.5 cm long by 5 cm thick.

The simple digital readout for a 20 μCi source for 60 seconds, without aperture would be

air 150,000 counts
water 61,000 counts
slurry 43,000 counts

for counting the pulse amplitudes in the 661-keV NaI(Tl) photopeak, from about 600 keV to 760 keV.

Bob S

 

[raj26]

Hi Bob

Thanks a lot for the info. I really appreciate your effort in giving a very detailed explanation every time. I can't be grateful enough.

 

[raj26]

Hi Bob

Regarding your previous calculation, i just had a few questions:

a) You showed the sample calculation with 1mCi source strength, but you recommend me 20 μCi for my application? Can i use 1mCi ?. Is this due to exposure concern?

b) What beam thickness i am employing? Can i employ a circular aperture?

c) What should be the voltage supply for my PMT?

d) For the digital amplifier, should i specify anything for that while ordering? Also can i get a digital reading of the height traversing?

Thanks
Rajesh

 

[Bob S]

My general concept of the Cs-137 NaI(Tl) layout is shown in the thumbnail. The Cs-137 source is above the slurry pipe. Directly below the pipe is a lead aperture to restrict the width of the beam to 40 mm, so as to limit gammas from thinner parts of the slurry pipe. This is a rectangular aperture, 40 mm by 75 mm. Below the lead is a 3" by 3" NaI(Tl) integrated scintillator photomultiplier PMT base assembly. It will have two connections: 1) negative high voltage input, and 2) analog negative polarity pulse output.

How long are the cables from this assembly to the electronics?

I don't know what country you live in, or if you have any brand names that you prefer, so I will just give you the name of one electronics supplier I am familiar with, Ortec. Who do you want to order from? Look at

http://www.ortec-online.com/Solutions/modular-electronic-instruments.aspx?tab=2

The minimum electronics would be
1) HV supply module (depends on PMT mounted on NaI)
2) Amplifier discriminator module
3) Counter timer module
4) NIM bin.

You would read the sum counts output on a LED display at the end of a 1 minute run. An alternate is to use an analog ratemeter like the model 661 ratemeter in the Ortec list (click on model number to see enlarged picture and data sheet). This analog meter readout will have an overall accuracy of ≈5%.

The minimum source size without the lead aperture is about 20 μCi. With the lead aperture, increase to 40 μCi. If the final separation is more than 200 mm, increase source strength by the square of the distance ratio. You could easily order a 100 μCi source which will cover all uncertainties. At this time with the 100 mm pipe, a 1 mCi source is overkill. If you have radiation safety agencies in your country, they will recommend that you stay several meters away. As I recall, 25 mm of lead will reduce the gamma radiation by a factor of 10. i suspect they will recommend about 100 mm of lead around the source.

Bob S

 

[raj26]

Hi Bob

I forgot to tell you that i have to place the density gauge parallel to the pipe(not above the pipe). This is because my slurry is inside a horizontal rotating wheel tube. Then, can i use the same layout or does it change the specs?

I live in Canada. Just to be clear to myself, are you recommending a lead shielding (of 100mm) around the source in addition to the lead aperture?

 

[raj26]

Since my wheel tube is rotating, i can't clamp anything to it. So does your recommended layout still work?

 

[Bob S]

You can mount the assembly either vertically or horizontally. I didn't use lead shielding when I was young. But we live in a different world now. Somebody in your province will regulate what you can do. Do you have any preference for the suppliers? Do you live near any national labs, like TRIUMF, Chalk River, etc.?

Bob S

 

[raj26]

I need not worry too much about the suppliers, as my boss will take care of it. I just need to give him the specs . Since my wheel tube is rotating, i can't mount the assembly on the tube. So does your recommended layout still work?

 

[Bob S]

Yes, leave ≈ 10 mm clearance on each side. You don't want to bump either the Cs-137 source or the NaI(Tl) crystal.

Bob S

 

[raj26]

Hi Bob

I have ordered my setup and the gamma ray densitometer for my experiments, and i am eagerly waiting for it to come.

In the meanwhile i am thinking of using a computer data acquisition for my gamma ray gauge. What all specifics i need to give it to my supplier. Basically i want the number of counts to be logged into a PC and i will interpret that in terms of local slurry density.

Thanks again for sharing your expertise.

Rajesh