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Offline neilep

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Lead....why is it so dense ?
« on: 31/01/2007 19:36:52 »
Lead seems to be pretty invulnerable to most things radioactive......why's that then ?

Is there anything that can hurt it?..or get through it ?..at reasonable levels ?...not supernova levels !!


 

Offline daveshorts

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Lead....why is it so dense ?
« Reply #1 on: 01/02/2007 11:58:38 »
I am not sure that lead is invulnerable to radiation, more that it is good at stopping it. Particularly at stopping gamma rays. The main reason lead is good for this is that it is both dense and has a large nucleus, so there is a lot of stuff in the way, there are some subtleties though.

 Gamma rays are hard to stop because they have too much energy for normal electrons to absorb, the best way to absorb them is to have a nice big nucleus which can absorb lots of differeny energies some of which will probably be close to the energy of your gamma ray.

Alpha particles are helium nuclei and have a large charge so will interact with things strongly and are hence very easily shielded with a piece of paper.

Beta particles are high energy electrons which are relaively easily stopped with a piece of wood or so.

  Neutrons are realy hard to stop because they will only interact with the neuclei of atoms, and atoms are 99.99% empty space, so you need lots of nuclei in the way to stop them, it is also best to have a nuclei that wants to absorb a neutron so capturing it. Boron is very good at this, and lead is bad.

High energy protons, these are only really a problem in space,because the solar wind is made up largely of them. These are actually best stopped by other protons, because if they hit a heavy nucleus they just bounce off like a tennis ball from a raquet attached to your heavy arm, but if they hit something the same mass their energy is shared between them like a ball hitting a raquet that is held loosely, so the proton looses loads of energy. This means that the best thing for shielding is other protons, so water is good, or polyethene.

So basically lead is very good for gamma and X-rays, but not much else
 

Offline lightarrow

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Lead....why is it so dense ?
« Reply #2 on: 01/02/2007 13:25:11 »
Lead seems to be pretty invulnerable to most things radioactive......why's that then ?
Is there anything that can hurt it?..or get through it ?..at reasonable levels ?...not supernova levels !!
Lead is good for this purpose, in addition to what Dave said, even because it has a high density; this means nuclei are more "packed" together and their average distance is low, so an X or gamma photon has more probability to collide with one of them. So, it could be better used Gold for this, (or Osmium, or Iridium) because it's much more dense than lead (almost double dense), but using it just to make a shield wouldn't be exactly economically convenient! (The same with Osmium and Iridium). Even Mercury is denser than lead (13.6 mercury and 11 lead), but it's much more expensive and toxic.

Some times those shields are made of concrete (they have to be much thicker than those made of lead), also to save money.
« Last Edit: 01/02/2007 13:29:48 by lightarrow »
 

Offline that mad man

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Lead....why is it so dense ?
« Reply #3 on: 21/02/2007 00:04:08 »
One thing I would like to ask: Does the Lead or Gold become radioactive over time from the radiation?

TMM
 

Offline daveshorts

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Lead....why is it so dense ?
« Reply #4 on: 21/02/2007 10:21:11 »
I think it will depend on the type of radiation.

beta particles are charged, but are electrons so it is very difficult for them to interact with nuclei.

alpha particles are positively charged, so are repelled by nuclei, so the chances of them hitting one square on and having any effect are going to be miniscule.

gamma rays are basically high energy light, these could be absorbed by lead or gold if they were of the right frequency. If the nucleus was increadibly unstable, this energy could encourage it to decay, but normal isotopes of lead and gold are stable so it would probably just be reradiated as a couple of photons at a lower frequency.

neutrons on the other hand, are uncharged so can get right into a nucleus, giving it more energy and another neutron. This will make a heavier isotope of lead or gold, which could easily be unstable and therefore radioactive.

eg
197Au + n --> 198Au  (half life 2.7 days)
 

Offline that mad man

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Lead....why is it so dense ?
« Reply #5 on: 21/02/2007 17:32:21 »
Thanks daveshorts.

I was reminded of the storyline in the James Bond film "Goldfinger" where the gold in Fort Knox was to be irradiated by a dirty bomb made of cobalt and iodine. The bomb was meant to irradiate the Gold and render it useless for 58 years. Not sure why it was 58 years though.

But that was fun science fiction!

TMM
 

another_someone

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Lead....why is it so dense ?
« Reply #6 on: 21/02/2007 18:38:05 »
Lead is good for this purpose, in addition to what Dave said, even because it has a high density; this means nuclei are more "packed" together and their average distance is low, so an X or gamma photon has more probability to collide with one of them. So, it could be better used Gold for this, (or Osmium, or Iridium) because it's much more dense than lead (almost double dense), but using it just to make a shield wouldn't be exactly economically convenient! (The same with Osmium and Iridium). Even Mercury is denser than lead (13.6 mercury and 11 lead), but it's much more expensive and toxic.

Some times those shields are made of concrete (they have to be much thicker than those made of lead), also to save money.

Regarding the effectiveness of these elements for stopping nuetrons (not electrons, positrons, or gamma rays), would it not be at least as effective to use a lighter element, such as lithium.

Lithium has fewer nucleons in its nuceus, and so has a lighter atom, and so a lower mass density.

On the other hand, lithium also has fewer electrons between the nuclei, allowing closer apscing between nuclei.

http://www.webelements.com/webelements/elements/text/Li/xtal.html
Quote
    * Space group: Im-3m (Space group number: 229)
    * Structure: bcc (body-centred cubic)
    * Cell parameters:
          o a: 351 pm
          o b: 351 pm
          o c: 351 pm
          o α: 90.000
          o β: 90.000
          o γ: 90.000

The closest Li-Li separation is 304 pm implying a lithium metallic radius of 152 pm.

Compare this to lead

http://www.webelements.com/webelements/elements/text/Pb/xtal.html
Quote
    * Space group: Fm-3m (Space group number: 225)
    * Structure: ccp (cubic close-packed)
    * Cell parameters:
          o a: 495.08 pm
          o b: 495.08 pm
          o c: 495.08 pm
          o α: 90.000
          o β: 90.000
          o γ: 90.000

So, it seems to me, that the lithium nuclei are actually packed 30% closer than the lead nuclei, even if they contain only 4% of the number of nucleons within each nuclei.
 

Offline Bored chemist

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Lead....why is it so dense ?
« Reply #7 on: 21/02/2007 19:02:21 »
OK
A few points.
1 lead isn't very dense. Gold is roughly twice as dense, even though the atoms from which it's made weigh less than lead atoms. It's roughly as dense as palladium and palladium atoms are roughly half the weight of lead. The atoms in lead are not packed together very tightly (see the post above).
2 It makes good Xray shielding because it has a high nuclear charge. The large electron density near the nucleus interacts strongly with Xrays.
3 The relative stability of lead to nuclear reaction comes from its having so called "magic" numbers of nucleons (I'm not making this up) They correspond roughly to the complete shells of electrons in the inert gases.
 

jolly

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Lead....why is it so dense ?
« Reply #8 on: 24/02/2007 15:36:45 »
o.k
« Last Edit: 25/02/2007 12:05:15 by jolly »
 

Offline Bored chemist

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Lead....why is it so dense ?
« Reply #9 on: 24/02/2007 18:08:58 »
Err, since nobody here mentioned alloys before I can't help wondering why you thought they were relevant.
 

Offline anneparker

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Lead....why is it so dense ?
« Reply #10 on: 07/04/2011 12:53:40 »
Lead is so dense because there are some solid elements present in it which makes it so dense that can cause serious problem to our body. People are not taking much attention to it cause they know that somewhat or somehow they are addicted to it.

newbielink:http://www.answers.com/topic/lead-10 [nonactive]
 

Offline rosy

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Lead....why is it so dense ?
« Reply #11 on: 07/04/2011 14:37:31 »
Quote
Lead is so dense because there are some solid elements present in it which makes it so dense that can cause serious problem to our body. People are not taking much attention to it cause they know that somewhat or somehow they are addicted to it.

What? This is complete nonsense... "solid elements present in it"? What's that even supposed to mean?
 

Offline lightarrow

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Lead....why is it so dense ?
« Reply #12 on: 09/04/2011 13:18:39 »
Lead is so dense because there are some solid elements present in it which makes it so dense that can cause serious problem to our body. People are not taking much attention to it cause they know that somewhat or somehow they are addicted to it.

http://www.answers.com/topic/lead-10
No. Lead is so dense because lead nucleus is heavy. No other elements present in it (we are talking of pure lead element of course).
 

Offline Bored chemist

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Lead....why is it so dense ?
« Reply #13 on: 10/04/2011 10:33:24 »
Leads isn't very dense.
This whole thread is bizarre.
 

Offline lightarrow

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Lead....why is it so dense ?
« Reply #14 on: 10/04/2011 13:19:08 »
Leads isn't very dense.
I assume it's intended in the sense of being one of the more dense among common materials.
 

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Lead....why is it so dense ?
« Reply #14 on: 10/04/2011 13:19:08 »

 

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