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avatar: RadicalRadon
RadicalRadon
15 posts
Replied to RadicalRadon's post on June 6th, 2019

We recently got a LS6500. Running minivials and the standard size ones. Our Eckert and Ziegler brand new standards (H3 and C14 and BKGND) show that the machine is working perfectly, background counts of 60. When we have a blank vial with just the Meridian Quantum Gold cocktail our background shoots up to about 300 DPM, (not really an issue yet). BUT when we put either a blank swab (Q-tip head) or a blank swipe (standard I think 1 inch circle) the DPM increases to 1400 and 6000 respectively. These are brand new swipes and swabs they don't have any contamination on them. The DPM is the same in all isotope ranges we select to run on the machine. When I cut the swipe down to a more reasonable size the counts go down but I can't figure out why I would be getting super high counts when nothing is there. The base count for a full-size swipe is 6000, cut in half its 1900, in a quarter its 1200, 1200 if i cut it into a circle and stick it on the bottom of the vial. These are still insane numbers of DPM.

The large vials are 20ml and ive been filling them to 10-12 ml, the small ones are 8 and I've been filling them to 4ml

Do I just report that on a blank swipe the background is 6000? and hope that it's fine from there?

Seeing as the EPA free release limits for most non tritium Isotopes is only 1000 DPM I'd really rather not have results that tells me that I have background 6 times that amount.

We did have a tech come out and install and then fix the diaphram that broke on it, he troubleshot and said nothing was wrong with the machine and guessed that our cocktail was a bad batch or something, this doesn't make sense because vial with nothing but cocktail in it gets me a reasonable background (288 DPM) considering the plastic vial and all other noise that might be going on.

The first time I ran a blank swipe test I actually got 14,000 DPM, vertified on multiple ranges on the counter.

Ideas?

avatar: dpkleessr
dpkleessr
1116 posts
Replied to RadicalRadon's post on June 7th, 2019

If your background vial is already reading 60cpm then even that is a little high for a background vial.  In all of my almost 33 years of working in Beckman LS counters, background vials that high would seem to indicate some kind of other issue as I would normally see the counts down in the teens or 20's.

The Q-Tips that you use, are they on wooden sticks or are they on a plastic stick?  Actually, seeing a vial with only cocktail in it and getting 288 DPM is concerning to me too.  Backgrounds should NEVER be that high under any circumstances.

What would be more helpful for me in order to help you is what do the background numbers look like if you only count them in CPM mode with and without H# turned off.  After you get me that information we can go from there but I am definitely concerned with what you are seeing.

Don

avatar: RadicalRadon
RadicalRadon
15 posts
Replied to dpkleessr's post on June 7th, 2019

The Q-tips are wooden sticks. When we do backgrounds at the facility with a G-M pancake probe we do get counts of about 55 per minute. 

 

I just ran a sample with on the Wide setting with H# off and on

The samples were in this order:

Full swipe

2nd full swipe but stuck to the side of the vial

1/4 of the swipe

E&Z BKGND Standard

Blank vial, half full with Meridian Gold Star Quanta cocktail

 

H# on (CPM): 

B1: 7416, H#: 86.6

B2: 4921, H#: 87.6

B3: 1844, H#: 71.1

B4: 75, H#: 1.3

B5: 541, H#: 67.2

 

H# off (CPM)

B1: 7496

B2: 4959

B3: 1852

B4: 64

B5: 598

I took pictures of the data sheets and what the swipes look like:

https://m.imgur.com/a/6gR9feN

Edit: sorry for posting this three times I think the server went down right when I posted it or something.

avatar: dpkleessr
dpkleessr
1116 posts
Replied to RadicalRadon's post on June 7th, 2019

After looking at the data you posted here I believe that you may have a light leak which typically has something to do with the diaphragm not closing properly but just enough to trigger the sensors that it is closed.  There is no huge difference between the results with and without the H# turned on is what leads me to that conclusion.  The one caveat to counting ANY type of swipe or swab  is that unless the swipe is either laying flat on the bottom of the vial or completely emulsified is that the geometry of the how the swipe sits inside of the vial can affect the count rates.  It is what is known as beta self adsorption in that the filter material can actually block some of the counts getting to the PMT's within the 20ns time frame to be considered as coincident counting.  Your counts, however, are just way too high for background counts of anything which again is why I believe that you may have a light leak.  I guess that I also have to assume that this is a relatively new occurrence for you using this LSC.  Kind of looking towards that you are going to need to call someone in to service your counter especially with a background count of 288 with only cocktail in the vial.

Don

avatar: RadicalRadon
RadicalRadon
15 posts
Replied to dpkleessr's post on June 10th, 2019

There was a diaphram error that required a tech to come out and replace our diaphram sensors, and we got these counts both before and after this work was done.

avatar: RadicalRadon
RadicalRadon
15 posts
Replied to dpkleessr's post on June 10th, 2019

We just ran a test with our standards, theyre pretty much right on the money so I don't believe that its a light leak issue. Additionally when I cut a swipe down to size so that it lays on the bottom of the vial we still get counts in the thousands range, I don't remember exactly how many but it was about 2,000 and I threw that sample out already.

 

 

avatar: dpkleessr
dpkleessr
1116 posts
Replied to RadicalRadon's post on June 10th, 2019

OK, I would like you to try the following. Make up some blanks with your filter supports and swabs placed in a rack with a USER # card in place as if you were going to count the samples in CPM mode only without H#. Place them inside the sample changer base with the lid closed for overnight. First thing in the morning count the samples WITHOUT opening the lid and report back to me the results. I believe that I know what's happening but I need you to do this in order to verify my assumption.

Don

avatar: RadicalRadon
RadicalRadon
15 posts
Replied to dpkleessr's post on June 11th, 2019

I did that overnight, or at least with the blacks that I had been using. There wasn't a huge change. It did go down slightly, but a blank vial with only cocktail got 540 CPM and both I ran with blank swipes still have ~10,000 counts. 

 

I can try making new ones and seeing if that changes overnight?

avatar: RadicalRadon
RadicalRadon
15 posts
Replied to dpkleessr's post on June 14th, 2019

I made up 4 samples last night, a minivial with only cocktail, one with a swab head in it, a large vial with only cocktail, and a large vial with a swipe in it.

Last night the counts were

Mini with cocktail: 188 CPM, 56.5 H#

Mini with swab: 429 CPM. 63.5 H#

Large with cocktail: 506 CPM, 68.5 H#

Large with swipe: 2146 CPM, 86.9 H#

 

This morning my counts were

 

Mini with cocktail: 192 CPM, 62.7 H#

Mini with swab: 431 CPM, 62.7 H#

Large with cocktail: 497 CPM, 69 H#

Large with swipe: 2126 CPM. 86.5 H#

avatar: dpkleessr
dpkleessr
1116 posts
Replied to RadicalRadon's post on June 14th, 2019

Statistically, there is no difference between the numbers. Seeing background samples as high as you are observing falls into a few categories: 1.) a light leak, albeit a small one; 2.) a chemiluminescence event going on. Typically the counts die out after some time but so far that does not appear to be the case with your system; 3.) You may have a noisy HV power supply or a problem with the front end in general which could include your PMT's, their sockets which are also preamps, and the Coincidence board. Really tough to diagnose this from afar but the only other thing I would ask you to do is keep the samples in the dark and count them WITHOUT the H# turned on, in other words, CPM mode, no H# and then get back to me. That may help me figure out where to go next with this problem. I worked on LS counters for over 32 years and there were times where they were real hair pulling situations in order to figure out what's going on.

Don

avatar: RadicalRadon
RadicalRadon
15 posts
Replied to dpkleessr's post on June 14th, 2019

Without the H# on.

Minivials:

just cocktail: 528 CPM

with swab: 559 CPM

 

Large vials:

just cocktail: 562 CPM

With swipe: 2336 CPM

 

I did have the door to the room that the counter is in open a bunch today and had to mess around with the vials to get them in the correct order so they may have had a little light exposure. I can run them again on monday morning and see what happens.

avatar: RadicalRadon
RadicalRadon
15 posts
Replied to dpkleessr's post on June 17th, 2019

I reran the samples this morning, this is without H# on and only CPM.

the lumex numbers are around .10%

 

Minivials:

just cocktail: 542 CPM

with swab: 571 CPM

 

Large Vials:

just cocktail: 538 CPM

with swipe: 2747 CPM

avatar: dpkleessr
dpkleessr
1116 posts
Replied to RadicalRadon's post on June 17th, 2019

Are you using plastic or glass vials?  There is absolutely NO way that you should have that many counts coming from essentially blanks unless there is a small light leak or you are using plastic vials that may be flourescing.  I have seen it happen more than once where either the plastic vials and caps flouresced or just the plastic caps did.  Sometimes dark adapting the samples helped and sometimes it didn't.  That's why I asked about what kind of vials you are using.

Don

avatar: RadicalRadon
RadicalRadon
15 posts
Replied to dpkleessr's post on June 18th, 2019

theyre plastic, included in the startup kit from GMI

avatar: dpkleessr
dpkleessr
1116 posts
Replied to RadicalRadon's post on June 18th, 2019

Plastic vials were the absolute worst case scenario for me.  Also, could you just try counting the mini vial with coctail but NO lid?  Do NOT try to count the maxi vial without the lid as the the opening of the vial may allow the lead plug that holds the vial from above to either get stuck in the vial or actually go down in the vial in to the cocktail.  That would be indicative as to whether or not the lids are flourescing.  Have you contacted GCI about your issue?  I'd be surprised if they did not at least try to help you out with this.  I actually met them once and thought that they pretty much had their act together.  Just curious but where are you located?

Don

avatar: RadicalRadon
RadicalRadon
15 posts
Replied to dpkleessr's post on June 19th, 2019

As far as I can tell the lids aren't the culprite. I ran a few of the blanks I have made and the numbers dont change signifigantly, I'll run them again after a few hours in the dark and see if anything changes. We did order glass maxi vials that are coming in tomorrow so hopefully that will at least remove a variable, and I made sure the caps were of a different material from the ones we have now (polyEthelyne vs polypropalyne). We had the head tech come out and fix a diaphram a few weeks ago and he was just as confused as us, he thinks maybe its a bad batch of cocktail. Were in Albuquerque, does elevation affect these machines? we are about a mile up. 

avatar: dpkleessr
dpkleessr
1116 posts
Replied to RadicalRadon's post on June 19th, 2019
Altitude can have a minor effect on count rates but certainly not in the range of what you are seeing. Are there any other possible sources of radioactivity In the area where the counter is located? Does the sample changer bed face a window that allows bright sunlight to hit the counter? The smoked plastic lid cannot block that kind of light from having an effect on count rates. In a wide open window with an unquenched background standard you should have less than 100CPM. A vial with only your cocktail should have similar readings. One thing I did note from going over the entire string of posts is that when you put your filters swipes in the vial the counts get very high even when no sample is on the swipes. That is a definite sign that you after some kind of reaction taking place between the cocktail and your swipe. Have you contacted the cocktail manufacturer to see if they have had any other reports of issues with their product? I am making an assumption that the cocktail is relatively new because cocktail does go bad over time and that can lead to strange counting activity. When the GCI tech was on site, what kind of numbers did he get with his unquenched standards for efficiencies and background counts? Did he leave your site with the same problem you had before he got there? The thing that concerns me about the diaphragm replacement is that it can involve more than just replacing the diaphragm. Sometimes small tweaks to the elevator shaft positioning need to be done as well as insuring that the diaphragm sensors are set such that there is no bounce after the diaphragm sensor senses that it is closed. A not uncommon occurrence that can lead to light leaks. Finally, counting swabs or swipes while using H# is not recommended under any circumstances because of the beta self absorption effect of the support materials. It is always a problem with dealing with a group like the EPA that want results in DPM's but the accuracy of those results is tenuous at best. Let me know how the tests go with the glass vials. Don
avatar: RadicalRadon
RadicalRadon
15 posts
Replied to dpkleessr's post on June 20th, 2019

Our counter is basically in a repurposed closet without any windows. the only way light can get in is either the door being open or the Florescent lights being turned on but I keep them off. There is the monitor for the machine as well but I doubt that casts signifigant amounts of light. I contacted meridian who makes the cocktail, they said that there wasn't a known reaction. We are getting new cocktail in on monday so that will see if thats the cause. I ran a sample with glass vials and the same thing in plastic in the tritium range DPM:

plastic only cocktail: ~1000

plastic with blank swipe: ~6000

glass only cocktail: ~660

Glass with blank swipe: ~4000

 

Which makes me think that the plastic is effecting the results somehow. We got glass large vials, is there a way to run those without the lids to see if thats the culprite? 

 

On our standards, C14, H3, Background, the numbers are right on what ekert and zigler say they should be which leads me to believe its nothing wrong with the machine. 

avatar: dpkleessr
dpkleessr
1116 posts
Replied to RadicalRadon's post on June 20th, 2019

Yes, you can TRY to run the glass vials without the caps on. The only issue will be whether or not the vial detector sees the vial as being there without the lid in place. If it does not detect the vial there is one other thing that you can try. Carefully cut some black electrical tape to mask off the side edge of the cap and also cover the top too by cutting a round piece to cover it. Just make sure that there are no pieces of tape over hanging on the lid so that it does not get snagged on the diaphragm assembly. Service engineers actually carried a BLACK vial with them to help determine if there was an issue with the LS counters PMT's and sockets. Please try that and let me know how it goes but the counts you posted really has me thinking that you either have a cocktail or vial issue.

Don