J308

I don't think the conversion factor for that tube is available. I have had a look through all of my J tube data (attached) and whilst the tube is listed, there is no sensitivity information, which is pretty essential for calculating it's conversion factor. I'm not even sure there are any datasheets for the J tubes. There may be datasheets in Chinese or something, but I haven't come across any yet.

There are a couple of things that could be done to ascertain the conversion factor. One would be by performing your own sensitivity test using a Co60 source and a 2-inch thick lead chamber to perform the test in, but I suspect you don't have those. Neither do I. The other is to use some maths to work out the cathode surface area difference from a known tube of similar ilk. I'm going to make some assumptions for the sake of example. Let's assume the J308 and the J305 are made using the same materials. I know the J308 pictured is black, but let's just assume the J305 is the same. We have the specs for the size of the cathode for both the J305 and J308 so we can work out the difference and then use that number to work out the conversion factor.
The J305 has an effective diameter of 10mm and an effective length of 87mm. The J308 is 16mm and 118mm respectively. First we need to convert the diameter to circumference, then we can work out the surface area by multiplying the circumference by the length. The formula for this is circumference = π * diameter, or C = π · ø. Or just use an online calculator, such as this one.

After converting the diameters to circumferences we just then multiply the circumference by the length and we end up with the following:
J305 - 31.4159265 * 87 = 2,733.1856055
J308 - 50.2654825 * 118 = 5,931.326935

Then simply see how many times the J305 surface area goes into the J308 surface area:
5,931.326935 / 2,733.1856055 = 2.17011494

The J308's cathode surface area is 2.17 times greater than the J305, so (assuming all materials are consistent between tubes) if the conversion factor for the J305 is 123.152 cpm per µSv/hr, times that by 2.17 and we have 267.240 CPM per µSv/hr for the J308. Or 0.00374 µSv/hr per CPM.

0.00374 would be my best, slightly educated guess based on the tiny amount of information available. If the J305 background count is say, 20 CPM, then I would guess the background for the J308 being something in the region of about 44 CPM. What is your background count with the J308? I'm curious to know.

That is really about the best I can do with the lack of info on the J tubes. My method of working out the conversion factor from the surface area and using the difference between another tube is flimsy at best, but it's about all I can offer, and I may be completely wrong. It has piqued my interest now, so I may try and get a J308 to do some actual comparison tests which should tell me if I was on the right track or not.

I have ordered a J308 tube. It is on it's way from China, so I should have it in about a week or so, all being well. I'll do some comparison tests when it lands to see if I was somewhat correct with my calcs.

Bert490, I forgot to mention in my last post (although you have probably figured out by now) that the sensitivity is generally based on size, or surface area of a tube's cathode. Whilst it looks about 4x the size of a J305/M4011 it isn't 4x the sensitivity, as far as I know. Imagine placing 4 tubes in a square, then look on end to the tubes, so the look square. All (most?) of the cathode on the inside would be disregarded as the radiation will be hitting the cathode on the outside, not in the middle. The detection area would be as though the cathode was rolled out to make a flat oblong. With pancakes/SBT10a etc. the reason they are so sensitive is they have a massive detection surface. Hmm, how many LND-712 end windows would fit in a SI-8b window. A few for sure. I'll have to work it out and maybe have a play with those tubes at the weekend and see if the surface area is directly related to the sensitivity.

Alright, please completely disregard everything I said in my last but one post. It is completely wrong. I received my J308 tube and did some testing at the weekend.

For a start, the specs I have the the J-tubes is not correct and the comparison testing I performed was completely inconclusive. I feel there is no way the conversion factor for the J308 can be (even roughly) calculated from performing comparison tests with a J305. They act like very different tubes, and not just 'a more sensitive version of another.'

The dimensions I have the the J305 are incorrect. Using a vernier caliper I worked out the effective length of the J305 to be around 60mm, not 87mm. The entire tube (including metal connection caps) only measures 88.5mm! How they got an effective length of 87mm I don't know. I also estimated the effective length of the J308 to be around 96mm, not 118mm as stated, so that puts all my previous numbers out completely. The J308 is a glass tube with a black, opaque, heat shrink sleeve over it. That hinders my seeing the end of the tin oxide coating on the tube, so I estimated/guessed the non covered part to be about the same as the J305. Anyway, the numbers are all wrong in my previous post, and based on the comparison testing I did, I'm not going to recalculate the surface area as the surface area seems to only be a part of a much more complex reason for the difference in sensitivity.

The comparison testing was all over the place. Some sources were a small percentage more on the J308 than the J305, whereas other sources were massively higher percentage than the J305. Due to this I can't give any indication to the conversion factor of the J308. It appears that the J308 is somewhere between 1.6 and 2.9 times more sensitive than a J305.

The results of the testing are as follows: