B Is it possible to predict which trick will work for fusing heavy elements?

[alvarogz]

Just a simple and bald question. Is it possible to fuse heavy elements like uranium?. I mean, massive stars core fusion process stops at iron.

Please don’t blame me for my lack of scientific rigurosity.

Thank you in advance.

AGZ

 

[Bandersnatch]

Yes, it is, but the process is endothermic. In nature, it (apparently) only happens during core-collapse supernovae and mergers of neutron stars. These processes supply excess energy to fuse heavy nuclei. That's where all elements heavier than iron and nickel come from.

 

[mfb]

With particle accelerators you can fuse heavier nuclei - that is the production method for most of the superheavy nuclei beyond uranium we can produce (up to element 118 so far).
Fusing uranium with uranium doesn't lead to a nucleus, however - if you put in so much energy that the nuclei get close to each other everything is ripped apart into many small pieces.

 

[Bandersnatch]

Fusing uranium with uranium doesn't lead to a nucleus, however - if you put in so much energy that the nuclei get close to each other everything is ripped apart into many small pieces.

Interesting, thanks! Could you answer:
1) is this qualitatively different from what happens to those artificial super-heavy nuclei, i.e. that it's not creating an extremely short-lived nucleus, but is breaking apart before there's even a nucleus to speak of?
2) what is the heaviest pair of nuclei that can be fused together?
3) is there a simplistic way (suitable for the back of an envelope) to calculate 2) ?

 

[mfb]

It is different, yes. The nuclei of elements up to 118 lived for some microseconds or longer. The IAU has some definition what the minimal lifetime of a nucleus has to be, you can look it up - it is significantly shorter than a microsecond, but it is also significantly longer than the diameter of a nucleus divided by the speed of light, the typical timescale of a collision process.

We don't know what the heaviest possible pair is. The largest atomic number created so far was 118, but it is expected that some more are possible. The probability goes down with increasing atomic number, so it gets harder to make them. I don't think an envelope is sufficient here.

 

[Vanadium 50]

is there a simplistic way (suitable for the back of an envelope) to calculate 2) ?

No.

The problem is that for heavy elements you have a larger neutron/proton ratio than light elements. Iit keeps going up with Z. So the problem in making superheavy elements is that you don't have enough neutrons. So you have to resort to tricks to get past this - like using the anomalously neutron-rich nucleus Ca-48. But predicting which trick is actually going to work ahead of time is impossible. That's why you have to run the experiment.