Radiation leak test instruments

[Happy Days2021]

I would like to find out please what is the preferred instrument to leak test very low levels of leaks of Americium, say in school sources. I mean an instrument which could measure leaks of 0.0005 microcuries. There are so many instruments. Some people say to measure the gamma emissions, others say measure the alpha emissions? Would an HpGe be more efficient to lower levels than say a handheld gamma Geiger Counter or is an alpha/beta smear counter best? Thanks

 

[Vanadium 50]

This likely does not exist. You are looking for 18 Hz of decays, spread out over some area that you don't tell us. Most of these are alphas, which are stopped in a few inches of air. You won't see them. There's also a 50 keV x-ray, which will travel through air, but will likely be absorbed by the counter's outer material.

Can you tell us more about your application?

 

[Happy Days2021]

I am looking for an instrument which would be suitable for leak testing a very old smoke detector and want a detector to read down to the lowest levels possible of Americium. Which of the instruments from my post above are mot suitable?

 

[Vanadium 50]

None of them.

 

[Happy Days2021]

Can you tell me what instrument would be suitable then please, even to measure to 0.05 microcuries of leakage or lower?

 

[Vanadium 50]

This is so close to zero as to be close to impossible. I can think of ways to do this, but they would be very expensive: basically the first step would be to clean every exposed surface and then to survey what's left when the water/alcohol has evaporated.

You are essentially trying to find a slice of a banana based on its radioactivity. Doing this requires a major research lab, not a survey meter.

 

[russ_watters]

I am looking for an instrument which would be suitable for leak testing a very old smoke detector and want a detector to read down to the lowest levels possible of Americium.

This wording is very odd, so just to be clear; what you are describing is leaking radiation, not leaking Americium. Is that what you really want? And where did you get the precision you specified? What is the purpose of this investigation?

 

[Happy Days2021]

Yes what I am wanting to wipe test is to check for any leaking Americium. I have a very old smoke detector with Americium and what to check if any radiation has leaked from the source. I want to find out should I use an alpha or gamma reader? HpGe/alpha beta counter/handheld gamma geiger counter. Thanks all.

 

[Vanadium 50]

The cost to measure this will be many orders of magnitude more expensive than throwing this one away and buying another smoke detector.

 

[russ_watters]

Yes what I am wanting to wipe test is to check for any leaking Americium. I have a very old smoke detector with Americium and what to check if any radiation has leaked from the source.

Again: these are not the same thing.

I think the device you are looking for may be a mass spectrometer. Or it my be that you don't understand/know what you are looking for.

 

[Happy Days2021]

I have a very old smoke detector and I want to check that due to its age that none of the Americium has leaked from the source and hence I want to perform a wipe test and measure any emissions from it. I want to know the best instrument to use for measuring such low levels and do I measure the alpha or gamma emissions?

 

[Vanadium 50]

As said before, this isn't going to work.

A "good" smoke detector has a very weak source (think banana) and the radiation needs to pass through its enclosure, the intervening material and air, the sensor enclosure in order to be detected. If you have a "leaky" but still working smoke detector the path is almost unchanged. The only difference is that maybe a tiny amount of material is outside the enclosure - but so what if there is? It means the rate will increase a tiny bit. 5%? 1%? Not only is it hard to measure to this level of accuracy, you have nothing to compare to.

Russ is right - you don't want to do this with radiation. The best way I can see is not to look for Americium radiation but to look for Americium atoms. This is analytical chemistry. I'm guessing a good lab would charge you $20,000.

But suppose they told you that it was "leaky" - something that I would find unlikely if it was not also "damaged". What are you going to do? Throw away the old detector and buy a new one. That costs $20 or so.

Why pay $20,000 to determine you don't need to spend $20?

 

[gleem]

The NRC only requires a leak test for alpha sources greater than 10μCi. A leak test is performed by wiping the source with a suitable material (damp filter paper swab ) and determining the activity removed. If greater than 0.005μCi, the source must be removed from service.

Probably the best detector is a windowless gas flow proportional counter in which you place the wipe sample inside the detector's sensitive volume. This will give about a 50% counting efficiency and will have a low sensitivity to background radiation. You might be able to build one yourself.

 

[sophiecentaur]

I am looking for an instrument which would be suitable for leak testing a very old smoke detector and want a detector to read down to the lowest levels possible of Americium. Which of the instruments from my post above are mot suitable?

A new mode detector would be a lot cheaper than a Scientific Instrument.

 

[Happy Days2021]

The NRC only requires a leak test for alpha sources greater than 10μCi. A leak test is performed by wiping the source with a suitable material (damp filter paper swab ) and determining the activity removed. If greater than 0.005μCi, the source must be removed from service.

Probably the best detector is a windowless gas flow proportional counter in which you place the wipe sample inside the detector's sensitive volume. This will give about a 50% counting efficiency and will have a low sensitivity to background radiation. You might be able to build one yourself.

Thank you. I don't have a gas flow counter but I have access to a HpGe detector-would this be suitable for leak testing to 0.005 microcuries? Or would a handheld Gamma counter (geiger counter) be better. I don't understand why some people say measure the alpha emissions, but others say measure the gamma emissions.

 

[Happy Days2021]

he only difference is that maybe a tiny amount of material is outside the enclosure

Where would the accumulate outside the enclosure-do you mean on the outside of the actual source, or the outside of the ionisation chamber?

 

[gleem]

Thank you. I don't have a gas flow counter but I have access to a HpGe detector-would this be suitable for leak testing to 0.005 microcuries? Or would a handheld Gamma counter (geiger counter) be better. I don't understand why some people say measure the alpha emissions, but others say measure the gamma emissions.

GM? Maybe if you can get a thin Mylar window GM tube with a thickness of less than 4 mg/cm². You still would have significant absorption by the window. You could use a HpGe to detect the gammas but being only 5% of the α-decays your count rate would at best be 20X lower ( ~ 4 cps) not correcting for absorption by the housing.

Since the sources are around 1 uCi, if the HpGe is available why not just put your source next to it and see what the count rate is. and report back.

In any event, you will need to calibrate the detector to get a dpm/cpm estimate.

 

[Happy Days2021]

GM? Maybe if you can get a thin Mylar window GM tube with a thickness of less than 4 mg/cm². You still would have significant absorption by the window. You could use a HpGe to detect the gammas but being only 5% of the α-decays your count rate would at best be 20X lower ( ~ 4 cps) not correcting for absorption by the housing.

Since the sources are around 1 uCi, if the HpGe is available why not just put your source next to it and see what the count rate is. and report back.

In any event, you will need to calibrate the detector to get a dpm/cpm estimate.

What would be the point of doing this? Would this be better than taking a wipe of the surface and then checking for emissions?

Since the sources are around 1 uCi, if the HpGe is available why not just put your source next to it and see what the count rate is. and report back.

 

[gleem]

You know the source is emitting radiation. if the source is not leaking the wipe will have no activity.

 

[Happy Days2021]

You know the source is emitting radiation. if the source is not leaking the wipe will have no activity.

Thanks but would the count rate of the actual smoke alarm tell me if it was leaking or not?

 

[gleem]

No. How could you tell if the radiation was coming from the inside of the source or the outside.

 

[Happy Days2021]

If the wipe test was clear from around the outside of the ionisation chamber, then would that not prove all activity was from the source-would cpm actually tell you anything about a leaking source?

 

[gleem]

I do not know the actual construction of the fire detector sources but they must have a very thin covering to allow the alphas to penetrate. This covering might be fragile and touching it might disturb this covering. Any leaking would most likely plate out on the inside. Since it would not be advisable to touch or disturb this source a wipe of the inside of the chamber might tell you if it was leaking but would not provide information on how much. On the other hand, a negative wipe does not necessarily tell you if the source has leaked. I do not understand your need to determine if this source is leaking as it is not handled, produces no gaseous radioactive products, and is enclosed in a container if left intact provides adequate shielding.

 

[Vanadium 50]

I do not know the actual construction of the fire detector sources but they must have a very thin covering to allow the alphas to penetrate.

The source itself (not the ionization chamber) is a thin piece of foil. Americium is extracted from reactors and the oxide is produced. This is insoluable in water, which is good. It's plated with gold and the whole thing is rolled again and again until you get very thin foil. Gold leaf, really. Then the leaf is bonded to something more macroscopic so it can be handled.

The amount of americium is really small. I'd guess about 200 ng for modern smoke detectors. That's an n as in nano and not some other letter.

As far as "leaking", does gold leaf leak?

As far as finding a "leak", it's surely easier to look for micrograms or maybe even milligrams of gold than nanograms of Americium. But, as I mentioned upthread, it's easier still to run over to Home Depot and get another one.

 

[Happy Days2021]

Thanks all

I do not know the actual construction of the fire detector sources but they must have a very thin covering to allow the alphas to penetrate. This covering might be fragile and touching it might disturb this covering. Any leaking would most likely plate out on the inside. Since it would not be advisable to touch or disturb this source a wipe of the inside of the chamber might tell you if it was leaking but would not provide information on how much. On the other hand, a negative wipe does not necessarily tell you if the source has leaked. I do not understand your need to determine if this source is leaking as it is not handled, produces no gaseous radioactive products, and is enclosed in a container if left intact provides adequate shielding.

Why would a negative wipe not tell you if it had leaked, surely negative wipe=no leak? Also would an HpGe not tell you the amount which had leaked on a wipe?

 

[gleem]

Why would a negative wipe not tell you if it had leaked, surely negative wipe=no leak? Also would an HpGe not tell you the amount which had leaked on a wipe?

Assuming you did not wipe the actual source a negative wipe just tells you that there is no activity on the surfaces that you wiped. The detector will only tell you the counts per second but not the activity of the wipe sample. The counts have to be corrected for the efficiency of the system. The source in the smoke detector
was not meant to be handled (wiped?). Assuming you only wiped areas accessible without dismantling the chamber i.e. the outside, you need to find a certified Am241 source to calibrate the detector to determine if any removed activity is significant.

 

[Happy Days2021]

Assuming you did not wipe the actual source a negative wipe just tells you that there is no activity on the surfaces that you wiped. The detector will only tell you the counts per second but not the activity of the wipe sample. The counts have to be corrected for the efficiency of the system. The source in the smoke detector
was not meant to be handled (wiped?). Assuming you only wiped areas accessible without dismantling the chamber i.e. the outside, you need to find a certified Am241 source to calibrate the detector to determine if any removed activity is significant.

Ok thanks...so will an HpGe always need a source to calibrate it, or can it not be calibrated and still show up activity?

 

[gleem]

It will show activity but you will not know if it is significant. And by the way, have you ever performed any radiation counting experiments before? Do you know how to evaluate the significance of your reading?

 

[rbelli1]

A new mode detector would be a lot cheaper than a Scientific Instrument.

Also if this detector is 10 or more years old it should be considered EOL and replaced anyway. Using it after any amount of testing should be considered a hazard to your life.

BoB

 

[sophiecentaur]

Also if this detector is 10 or more years old it should be considered EOL and replaced anyway. Using it after any amount of testing should be considered a hazard to your life.

BoB

Especially for a member of teaching staff in a UK school. Radioactivity is one of those things that kids really don't need hands - on experience of. There are so many other parts of the GCSE and A level course that are hassle free and can involve student participation at a concrete level.
Also, because radioactivity scares the pants of parents and senior (non-Scientist) members of staff, you can find yourself in hot water if all the i's are not dotted and t's crossed.
I remember watching a head of Science giving a demo of ' bromine behaviour' which involved dressing up (teacher only, of course) and warning the kids that it was extremely dangerous. What a needless risk to a class of thirty kids (also him and me) and to what end? I think it could have been a testosterone thing for him.