Old lead

How to build the perfect shield for your measurements? Discuss it here!
Alain
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Re: Old lead

Post by Alain » 07 Jul 2018, 17:28

Maybe it seems unusual, but I made inner liner from steel sheets (I think 3 or 4 mm thick), which I selected before to be non-magnetized. It works good for me, Pb fluorescence is almost negliggible. Primary reason for such construction was because this inner chamber serves as a support structure for lead plates, which are loaded around in vertical position and also as a barier against radon (I blow a small flow of fresh air into the chamber to keep radon away). If you will make good shield, you may actualy see the difference in background when measuring without air exchange or with it (but this depends on how much radon is in the room, where the shield is located - and external air should be from the outside, of course).
Regarding tin, I have 70 kg of tin ingots for eventual use in my future shields. Since the required thicknes of tin layer should be only a few mm, casting directly into the shield is out of the question. My idea is centrifugal casting in a big steel pipe, used as a mold (casted iron pipes are made with a same technique). Another way, which actually works, would be pouring melted tin on a (copper) metal plate or sheet and as soon it solidifies, flush with water quickly to cool it down. Due to different thermal expansion coefficient, tin peels-off from the copper and you get some sort of rough tin sheet, not homogenous in thickness and with wave surface, of course. Lower metal sheet should not be shiny clean, it is better to be oxidized (but clean from dirt), in other case tin could permanently stick (solder) on it, especially if temperature is too high.
It would be interesting too see, how did you casted rings and what dimensions they are. Are they flat or with a step-edge for overlapping when loading one on another?
I sincerely hope that you stick to all preventive measures when melting and casting lead. Wear all protective equipment for face and body, gloves, mask etc. Molten lead can explode vigorously when pouring in molds which are not 100% dry and pre-heated. Burns can be very heavy and setting fire of surrounding combustible materials is not uncommon. Dirt and oxide layer floating on melted lead are toxic, as you know, fumes also. Wearing a mask is mandatory. Fine, invisible lead-oxide dust can float in the air and gets everywhere, also in (and on) your body. I hope you are doing this in a workshop or outside, not in an apartment. Wash yourself thoroughly after each work and keep working clothes separately from others.

Looking forward to see your results :)
Last edited by Alain on 09 Jul 2018, 01:32, edited 3 times in total.
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Alain

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pilgrim
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Re: Old lead

Post by pilgrim » 07 Jul 2018, 22:01

Hi Alain,
I cast the lead in Green Sand, a mixture of sand and bentonite (15-20% bentonite).
I created the mold around a steel ring as a positive, then, once removed the ring I poured the fused lead into the mold.
The inner ring's diameter is 175mm and 215mm the external.
After the castings I machined the rings surfaces with the lathe to grant the coupling between rings; the step-edge is only presente between the bottom (a simple disc 25mm thick) and the first ring.
Obviously I wore a FFP3 mask, protection glasses and gloves, and had a shower ;-) after the main castings.
These operations, not in apartment but in my garage, with a fan pointed on the pot containing the lead.

For the tin side of the liner I thought to the common tin wire for welding (the RoHS one), I thought to wrap it around the copper cilinder, to obtain a single body, tin + copper.
The tin in sheets that I find on Internet is too much expensive, so I decided to try this solution, that is always removable.

Right now I'm taking a spectra of the background inside the shield without any liner, to have a comparison when I will assemble the liner.

Thanks again for your interest.
Daniel, Italy

Alain
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Re: Old lead

Post by Alain » 07 Jul 2018, 22:46

Hi Daniel,

Very nice to see that you took all safety and precaution measures :)) You probably know how nasty thing lead is and how much damage caused in the environment.
I see that you have some foundry knowledge and skils, very good:) Yes, I'm familiar with that type of sand, but be careful to clean casted pieces good or machine surface layer away (as you do on the lathe), some unwanted minerals can be hidden in Green sand and cause surface contamination, consequently rise the background. But I think bentonite clay should be quite clean (just as a curiosity: bentonite, montmorillonite and illite clay minerals have very high affinity to caesium (unortunatelly Cs-137 also), especially illite and that is a main reason, why Cs-137 (and Cs-134) "sticks" so much to the soil after contamination event like Chernobyl or Fukushima and stays there for decades, despite the fact that caesium compounds are very soluble in water and rain could easily wash caesium deeper into ground). Nevertheless, precaution is always good.
Stepped edge or overlap is not useful only because easier stacking of rings and the mechanical stability of such shield, but also because it assures additional barier from the outside radiation to the detector through imperfect surface of the rings, which causes some tiny slit, crevaces or holes between rings. If you make intermediate step, radiation cannot go through in a wiggled way, only straight line, therefore it can tolerate such imperfections (or later damages due to softness of lead). For the same reason also lead bricks are usualy made of such "broken line" shapes. But usually, if rings are smoothed and plain, this aperture and solid angle of the slit would be so small, that even without stepped edge this effect should be undetectable.
One important thing about lead casting is that it is quite prone to make internal voids ("lunkers"), especially when bigger castings are made, mainly for two reasons: as almost every molten metal lead has also a tendency to shrink when cools and solidifies. If solidifying takes place first at outer surfaces (which is the case at big castings) and without suplying additional liquid metal into casting during solidifying, internal voids are very likely to occur. And it is not easy to discover them with X-rays, like in steel castings, because of great stopping power of lead for X-rays. Second reason for voids and gas bubbles in lead castings may be gases, which escapes from mold during casting proces due to elevated temperatures (moisture, dirt...). Therefore it is important, that molds are clean, dry and preheated.
Personally, I think tin intermediate layer is no of such importance regarding its high price and difficulties. I think steel layer would also suffice (in case steel is not contaminated, which at steel unfortunatelly you can never be sure) and even most inner copper layer would do its job, if proper thick. Another possibility would be zinc sheet, which is not expensive and could be found at roofing craftsmans or in graphic industry (plates are used in some special printing machines and then recycled). But zinc, as the nearest neighbour of copper in the periodic table, behaves practicaly the same as copper in radiation shielding practice, so I think this suggestion is of low value.

We are happy about your progress and looking forward to see your spectra :)

Alain
Last edited by Alain on 09 Jul 2018, 01:30, edited 1 time in total.
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Alain

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pilgrim
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Re: Old lead

Post by pilgrim » 08 Jul 2018, 01:48

I'm used to take spectra in the basement, to have lower background and to have minimum temperature differences between day and night.
Actually it's the place with higher Radon concentration (checked with a Corentium, i have 300 Bq/mq in a couple of day), but I always leave the windows open to keep the level around 50 Bq/mq.
So I don't think such levels could contaminate surfaces, at least in detectable quantities.

Thank you for your very interesting informations about sand and bentonite.
Together with the informations about old lead written by Max Gaspa some posts ago, It looks like I had a very bad idea using this lead! :-(
In fact, if I have to to do a shield again, I wouldn't melt it no more but I would do it the same way Max did (viewtopic.php?f=10&t=341)
No risk for health and less hard work to do!

In some rings happened what you described (lunkers), but only on the surface...
Maybe in the future I will melt these rings again, but for now I had enough of melting lead! ;-)

Thanks again,
and keep in touch.
Daniel, Italy

Alain
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Re: Old lead

Post by Alain » 08 Jul 2018, 06:22

Daniel, you did fine job, I don't know why it would be the bad idea? Maybe we "scared" you too much with our recomendations (especially me, sorry;) I think shield will do its job and you learned a lot of new skills (casting isn't so easy, as it seems). What can be wrong here? :-)

Regarding radon: very good, that you have control and you aware of this problem. From my personal experience I can tell you, that even in a building (2nd floor and not old building, I didn't measure radon directly, but from other indications it was low to modest) I noticed this effect and purging shield with clean air helped to lower the background. Not much, but effect was visible. In school, where we made another shield and spectrometer, we got a bad luck with selection of room, where spectrometer was situated (it was in the basmement floor because shield weighted over 500 kg and school building was 100 years old, so we wouldn't want to destroy it...). It was impossible to do decent spectra there because background start to rise as soon you closed the door - and radon progenies were clearly visible in spectra. When we done measurements of radon, it showed astonishing 7000 Bq/m3 =( :-S We obviously got on the radon-spring, which we could use it for radon-spa tourism. Of course we moved spectrometer in the other room.

Maintaining stable temperature is wise move. Stable temperature is very important if not crucial, if you want doing long-term measurements. Most critycal is on my experience USB sound card (unlesss you have high grade or professsional one), then HV power supply (unless professional, but even they are not 100% immune), and finally crystal and PMT also shows strong temperature dependance. Everything mentioned create spectrum shifting, broadening and lowering of peaks, resolution degradation etc. (and in case of measuring activities also heavily degrades that). I solved a lot of these problems when I put all sensitive electronics (SB, HV power supply, preamplifier) in a small metal cabinet where I maintained stable temperature with a small light bulb and a fan, bulb was switched on an off via temperature regulator. I easily reached temperature stability under 0,1 °C (couple of 1/1000 of degree, being precise, but that was sure an overkill, only my professional deformation :P). Wow, that was a revelation, all stability related problems with spectra practicaly gone! HV was stable at 0,1 V on a long-term basis (first spectra shifting become visible if HV deviates more than couple of tenths V). Room was air-conditioned, but despite that it had daily swing about 2 °C or more, so crystal and PMT wouldn't appeared to be happy with this at the first glance. But constant measurements of temperature inside the shield (directly on detector housing) showed stable conditions, so this apparent problem was overcomed simply by crude shield mass (thermal inertia) - shield weighted well over a tonne.

That are some of my experiences with shield, radon and temperature stability, I hope that you can use some of them. Now I don't have such troubles any more because underground, where detector is situated now, the temperature is inherently stable by some tenths of degree on a yearly basis without any air conditioning or similar active measures. Thank you, mother Earth ;-) But on the other side I must purge the chamber in the shield with fresh air from outside, because radon concentrations can be sometimes quite high underground.

I think you will build a good shield, but try to increase the thickness, this is the weakest point to me - now you have only 2 cm, according to diameters you posted. I know, you must have more lead for that, unfortunately :-(. But on the other hand, it is a good idea to make internal space big enough, which is your case. Many amateur shields suffer too tight internal chamber, where detector is literally touching chamber walls. Such systems suffer high backscaterring issues and this is not good for sensitive, low-level measurements of week samples. Thus, everyone must find his own compromise between internal diameter and wall thickness, based on quantity of lead, which is available for him.
Regarding maximum thicknes, probably someone alraedy answered: professional shields have between 10 and 15 cm lead wall thickness. More doesn't contribute much to background reduction (speaking for gamma-spectroscopy), only increases weight and costs. Even contrary is true - according to literature and my personal experiences, increasing thickness over some limit again start to increase background slightly. That is because you offer more and more volume(mass) to cosmic rays to interacts with (mainly hard component, muons and similar high-energetic particles). They have so high energy that you cannot stop them simply by few cm (or dm) of lead, because they go through like bullet through the butter and during interaction they create soft component (bremsstrahlung / new gamma, electrons...). So using common sense is good at building radiation shields. It helps a lot, when you are in the lowest floor of high building to supress cosmic component (therefore professional spectrometry labs are usually situated in a basements or lowest possible floor). Your decision about a basement here clearly comes to the point :-)

I was long, but I hope these information help someone.
Last edited by Alain on 08 Jul 2018, 17:45, edited 1 time in total.
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Alain

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pilgrim
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Re: Old lead

Post by pilgrim » 08 Jul 2018, 16:12

Good morning Alain,
many thanks for your post, very clear and interesting, and for time you dedicated to it.
When the liner will be finished, probalbly I'll buy other lead, but in sheets this time! To wrap around the existing shield, to increase the thickness keeping it demountable.
Probably at the end of this year or next : matter of budget € € :-) :-) :-)
Daniel, Italy

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pilgrim
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Re: Old lead

Post by pilgrim » 08 Jul 2018, 21:17

Ok, this morning I put into the castle the copper sheet, using the rings to keep it rolled, after a little bit of struggling now the castle is mounted again.
The sheet is taller than the castle, so it juts out few cm, but in a future non so far, I will cast another ring and the top of the castle to close it.

Here an image of the copper before closing the castle, kept closed in a posh way!
Cu_Liner.jpg
Daniel, Italy

Alain
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Re: Old lead

Post by Alain » 08 Jul 2018, 22:17

Nice finish of rings:) How did you machined them in a lathe, regarding softness of lead? Three or four-teeth chuck?
Last edited by Alain on 09 Jul 2018, 01:25, edited 1 time in total.
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Alain

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pilgrim
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Re: Old lead

Post by pilgrim » 09 Jul 2018, 00:05

Good question!
The clamping on the spindle's clamps must not be too much strong or there will be mechanical deformations.
Then low rpm and lot of coolant.
I've only machined the surfaces of contact, not the external and the internal diameter, that resulted awful!
Finally I used an abrasive stone as final machining, a kind of grinding, to reduce as much as possilbe fissures.
The step-edge would be ideal, but for now it's enough to make some test!
Daniel, Italy

Alain
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Re: Old lead

Post by Alain » 09 Jul 2018, 01:24

Excellent work:) Good to know your experience and good practice in lead machining for any future work of forum members (including me;)
Regards,
Alain

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