cobalt concerns

By CWI555

Date 11-05-2007 06:00

Run away

By CWI555

Date 11-05-2007 06:11 Edited 11-05-2007 06:14

No seriously, Cobalt is more penetrating than Iridium. IR192. You could probably get a shot on the 2.25" but not on the 4" with Ir192. For economic purposes, the cobalt makes sense as your shot times will be considerably higher with IR, and will not likely get the thicker sections. Cobalt puts out around 14.5 R/hr per curie at 1', whereas IR puts out 5.9R/hr at 1'. Co also has lower hvl values than the Ir so it takes more shielding or longer distance to work safely. Time, distance, shielding, or Time, distance, assistant, as some of the less scrupulous radiographers will call it. Between the lower hvl's and the higher initial energy, your main concern will be making sure no one's around. It's not something you want to try and set up in a corner while people are still working. The shot on an apples to apples basis would have a little bit higher subject contrast.
Other than that, no real differences. Zoomies are zoomies.
(edit) there is a difference in the projector size. You may need to make arrangements for a fork lift depending on the device.

By darren

Date 11-05-2007 09:14

they are talking about digging a pit and putting cement blocks around it but i wonder if that is enough.
we will have to see what really transpires before i make any judgments on whether i'm safe or not.
i would like one of those little exposure badges though just so i know i am not getting inadvertently exposed.
i work nights and thats when we do all out "bombing" so i want get a handle on this before we start to shoot

By js55

Date 11-05-2007 14:29

We used a lot of Co in a power shop I worked in (the thicknesses made it economical), but we had it in an encloser with leaded walls 4 feet thick and 10 feet high.
We had a 50,000 lb positioner situated with its center about 10 feet off the ground in front of a pit to handle long branches, and about 50 feet away from the encloser and we still deactivated the welder when poppin the pill. My experience is that it can be handled safely but with great respect as Gerald indicated.

By darren

Date 11-05-2007 19:21

i am interested as to why the radiograph tech always turns of our welder when he is doing a shot?
thought he was just being a pain but maybe you could inform me otherwise.
thanks
darren

By hogan

Date 11-05-2007 14:33

the only other thing would be the actual size of the source. watch the ug

By CWI555

Date 11-05-2007 17:47

The HVL for average concrete with cobalt is 2.4". The same for Ir. is 1.75". So if you started with 100 ci of Cobalt, two HVL would bring that down to effective 25ci. on the other side of the wall. Cement blocks if they are of the brand normal for plants that pour excess concrete to blocks is around 30 inches. Using the linear values makes it 12.5 HVL.
However; it is not true linear rather it's effectively logarithmic at around the 3rd HVL as the lower energy's emitted by the cobalt are attenuated/scatted at a higher rate.
Therefore without doing some calcs I place it at about 14 to 15 HVL realistic. That will make it about .003ci @ 1ft. walk that out to around 10 ft using inverse square law, that puts it at .69mr on the outside surface of the concrete. On a straight linear it's no problem. The concrete needs to be laid out so that the gaps are overlapped or filled one.
With that out of the way, there is the concern of scatter from the roof of your structure. I don't know the the height of your structure, but that is typically mitigated by using a 10HVL collimator that when combined with proper use and the distance to the nearest scattering surface (typically 20 plus feet) should mean you have no problem.
I wouldn't worry about "bombed" if I were you under those conditions.

As for the badge, you have to be under their monitoring program to have one of those which is exactly what you don't want. The public is limited to 2mr per hour with a weekly max of 100mr in the states. If you are being monitored officially, that goes to 5000mr a year. I would just ask them to show you at their first crank out that the boundary is sufficient.

In short, I don't see a problem with what they propose.
besides, a little glow at night makes life exciting.

Regards,
Gerald

By hogan

Date 11-05-2007 18:01

is this a panoramic shot or directional? there are collimator's for cobalt.

By darren

Date 11-05-2007 19:19

there will be all sorts of shots,
awesome info gerald, we will use one of those meters that tell us what the exposure is, 2 mr/hr is what we were told as well.
the gaps in the concrete is something i'm concerned about as well.
i will bring this up with our iso guy and some of the people that are concerned with it.
i will keep the forum informed as best i can
thanks
darren

By CWI555

Date 11-06-2007 00:11

Keep in mind, that is 2mr in "any one hour". You can expect a spike at crank in and crank out. Absorbed dose is a factor of impinging energy level and time in that energy level.

You could get hit with a 1000mr field for 7 seconds and still be under the required limit of recieved/absorbed dose.

By thirdeye (***)

Date 11-06-2007 01:28

Hey Gerald,

"As for the badge, you have to be under their monitoring program to have one of those which is exactly what you don't want. The public is limited to 2mr per hour with a weekly max of 100mr in the states. If you are being monitored officially, that goes to 5000mr a year. I would just ask them to show you at their first crank out that the boundary is sufficient".

Not trying to hijack this thread, just curious...If there was a possibility of you receiving any exposure why would you not want to be monitored? Also, I believe that the dose limit for the general public (from licensee operations) is 100mr/year, not per week.

Do you think it would be a good idea for the shop to invest in some area monitors or possibly personal monitors to prove they are buying in to the ALARA philosophy? Even though we monitored exposure rate at the boundries, we used to post dosimeters in a couple of locations just to document exposure during a shift.

~thirdeye~

By CWI555

Date 11-06-2007 03:20 Edited 11-06-2007 03:24

opps got ahead of myself, you are correct it is by the year.

As for the rest of it, Monitoring yourself via a film badge/tld would be more trouble than it's worth. I can see maybe area monitors, even hanging a few SRD's if you want to proof ALARA.
If your just to paranoid about it, then use a group such as this one.
http://www.stanforddosimetry.com/badge_service.html?gclid=CITNsLubx48CFRk0awodzxHDYQ
It's the reporting required for TLD's and film badges that can be problematic. I've experienced people who have done this, and have had the following happen:

Entering my ropes because they "think" wearing that badge makes it ok to come on in.
Having a panic fit when an area monitor went of when cranking in a vault and insisting they had been "irradiated", and trying to make my company pay for the emergency read.

Then there are the potential problems:
Leaving the badge in an area where it will be exposed during normal radiographic operations
Leaving it in the car day after day in the heat (more true for film type badges)
Leaving it on during medical x rays without thinking.
all of which can end up with a recorded dose level.

Then it's time to explain how and where the radiation came from. Being monitored means being recorded for the record.
All in all, I think it's a bad idea. Not to mention what happens if a recorded dose in excess allowable limits for the public happens to get back to your insurance company.
Non permanent record devices such as area monitors, survey meters, etc maybe.

By thirdeye (***)

Date 11-06-2007 05:40

Okay, now I see your point in a different light. Radiographers usually do have more respect for proper use of their badges. Some of your examples could be awkward to say the least when it comes to explaining (and documenting) them.

I was pretty lucky. In 27 years I had mine fall off once near a shooting rack one time, taking a zap around 1R in about an hour of shooting. I only lost one badge and washed (the film type) 2 or 3 times.

~thirdeye~

By darren

Date 11-06-2007 10:55

we work with the current 'bomber' fairly well we have access to a meter if we want. i am curious as to what/who/why established the limits.
doesnt welding give off gamma and xrays and all that good stuff.
i know when welding on an radio astro observatory they checked (listened with some funky home made jethro bodine kinda equipment)to all of our welding machines and only a few passed and there was no rhyme or reason an old smokey one and a new one were ok and others that were new and some that were old were not.
our biggest concern is the new cobalt source and the people running it, we meet them soon and will find out what is going to happen. right now they are proposing shooting on weekends when there is no one else in the shop. this will help us but the neighbors might not like it. ill assume with the source goes a lot of responsibility so i am most sure it will be handled responsibly.
is there a site or info on the epidemiology and pathology of people within the industry.

By CWI555

Date 11-06-2007 16:17

The DOE and NRC established limits. There is a difference between natural background, industrial background, and occupational exposure. Jet pilots for instance typically have the highest non-occupational exposure as they typically fly above 15K and lose some of the benifit from atmospheric shielding of cosmic radiation.
A CRT telivision will give off some radiation, welding will certianly do so, but all of these are accounted for in the exposure limits set by the NRC.
http://www.nrc.gov/reading-rm/doc-collections/cfr/part020/
10CFR part 20 is a good place to do some searching on the limits.
As to why and how those limits came to be, You have to trace the history all the way back to the curie, becquerel, and families/historical precedences such as the pacific atoll experiments etc.

By Joseph P. Kane (****)

Date 11-06-2007 21:13

Darren

Safety issue: If the radiography is done in accordance of the current safety regulatioons, the differences between IR 192 and Cobalt radiography will only be time, distance or shielding related. I recommend that the company have a safety review just before the work begins. Inadvertent or careless violations of the exclusion zone during exposure periods would likely give a bigger burn than a similiar violation when using Irridium.

If there is a source retraction problem on site, the delay caused by the recovery operation will probably be a lot longer than an similar IR 192 problem. However this is not likely to happen.

There is also a weight difference for the equipment, but with the thicknesses you would be involved with to use Cobalt, equipment for lifting the camera will be the only change.

The operator will establish the exclusion zone, which will probably be larger than the zone required for Irridium.

The film reading and number of trial shots, is more complex if you are looking for very small defects.

The test lab will charge you more.