# Search results

• Users: Grogs
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1. ### U-235 fission

You'll find this data at the NNDC useful for answering part of your question: http://www.nndc.bnl.gov/chart/reColor.jsp?newColor=235ufy" The colors show the relative frequency in which a given nuclide appears as a product of thermal fission of U-235. Notice that for thermal fission, the...
2. ### Half-life homework problem

It's not in the decay chain, but it is in the denominator of the equation used to find the fraction of U-235 in natural uranium: frac(U-235) = N(235) / [N(235) + N(238)] (Ignoring the really small amount of U-234) That's why most of the times I saw this type of question in basic physics...
3. ### Half-life homework problem

Your equation is right, although that .77 should either be .77% or .0077. I get something around half the value you reported when I plug the half-life and 1 Billion years into my calculator. One thing you're not accounting for is the decay of the U-238 though. Does the problem say to ignore...
4. ### Fulltime Jobs while doing a Phd

I think it's doable, it's just a matter of finding the right program, staying motivated, and being willing to make sacrifices. The right program is key because since you're a nine-to-fiver you need a program that offers classes in the evenings. The NE department at my school does really well...
5. ### I need to fit a tail-heavy Gaussian curve

Thanks, D H. I'll take a look at the exponential and see if it does any better. Another thought that occurred to me was that what I'm looking at may be the convolution of two different Gaussians - one with a small maximum and a very broad distribution and a second with a very large peak and a...
6. ### Nuclear Eng a good idea?

Hi, I'm finishing up my PhD in NE, so I'll try to offer you some insight. I do agree with your assessment of the workforce... getting older all the time, so there should be a lot of positions coming up. It's a subject that is mentioned frequently at conferences I go to. I got in for some of...
7. ### I need to fit a tail-heavy Gaussian curve

Hi, thanks for the quick response. The mean is zero. The data I'm modeling is essentially the angle a neutron scatters in the x-y plane, which is going to be a symmetric function. f(theta) is the number of counts I get in a detector at angle theta. The incoming particles I modeled are all...
8. ### I need to fit a tail-heavy Gaussian curve

I need to fit a tail-heavy "Gaussian" curve Hi, it's been a long time since I've been around PF. Homework Statement This isn't a homework problem per se, but I've been trying to fit some scattering data using a Gaussian function using a least squares approach and it's not working so...
9. ### Beam of particles, scattering

Cool problem. I would approach it this way: The microscopic cross-section is ~ the area presented by a single nucleus, so in this case \sigma = \pi*(3x10-15 m)2 = 2.83 x 10-29 m2 Once you have that, you can calculate the macroscopic cross section using \Sigma=N\sigma and then you can use...
10. ### Radioactive decay and finding half life

Your book didn't really do you any favors on the k thing. Since the amount of radioactive material remaining decreases as time increases, the first equation should properly be written as: \frac{dm}{dt}=-kt The minus sign on the kt represents the fact that m is decreasing as time increases...
11. ### Need help with volumes of spheres with holes.

timm3r, Draw a triangle with one vertex at the center of the sphere, the 2nd at the top wall of the cylinder, and the 3rd straight down from the second so that it forms a 90 degree angle with the first two. What is the length of the 3 sides of the triangle? That should give you your...
12. ### Nonhomogeneous Boundary Value Problem

Are you saying I should model u_{hom} and u_{part} as: u_{hom} = \sum \limits_{n=1} ^ {\infty} A_{n} \exp(-j_{n0}^2 t) J_{0}(j_{n0}r) (There is no n=0 solution for the homogeneous case) and u_{part} = c + \sum \limits_{n=1} ^ {\infty} A_{n} \exp(-j_{n0}^2 t) J_{0}(j_{n0}r) = c (there's only...
13. ### Nonhomogeneous Boundary Value Problem

Halls, The A_{n}'s are zero in that case. I should point that I goofed up when I wrote the homogeneous solution above. The n=0 case shouldn't be an eigenvalue (so no constants in the solution.) I must have been working on too many problems at once and gotten them mixed up. I edited my...
14. ### Nonhomogeneous Boundary Value Problem

I've got a nonhomogeneous BVP I'm trying to solve. Both my book and my professor tend to focus on the really hard cases and completely skipp over the easier ones like this, so I'm not really sure how to solve it. It's the heat equation in a disk (polar coordinates) with no angle dependence...
15. ### Need help with a couple of BVP's.

Halls, thanks for the reply. I'm not sure where you're getting that. This is the heat equation (u = temperature,) not the wave equation. It's 1st order in t -> 1 IC required. The 2 IC's I posted are for 2 different problems where everything is the same, save for the IC's. Hmm...
16. ### Need help with a couple of BVP's.

I've got a couple of BVP's I'm working on that are giving me some problems. First, I'm asked to solve the 1-D Heat Equation: k \frac{\partial^2 u}{\partial x^2} = \frac{\partial u}{\partial t} in a thin rod of length L subject to the following: BC1) u(0, t) = 0 BC2) u(L, t) = 0 IC) 2...
17. ### Newb in nuclear

They had a great show on this recently. It was one of the 'Ten Days That Changed the World' on the History Channel. It started with Teller and Szilard driving to Einstein's house to ask if he'd sign the first letter, then progressed through the first and second letters, the CP-1 reactor, and...
18. ### How can i prepare for a medical school?

In the US at least, you can get student loans to cover that. The typical doctor probably comes out of Med school with \$150K or more in student loans. As far as what you should take in college, take a look at some med schools you think you might want to attend and see what courses they...
19. ### Newb in nuclear

Hahn and Strassman were the first to observe nuclear fission in 1939. Fermi and Szilard built the first self-sustaining nuclear reactor in December 1942.
20. ### Electron / muon

To echo Candyman, I don't see a definite limitation there. The biggest problem I see with power transmission is the loss of energy in the form of heat due to resistance in the wire. Potentially, we can (nearly) eliminate that problem through the use of super-conducting materials, so high...
21. ### Nuclear reactor physics by Weston Stacey

If you haven't seen it, this is a pretty useful site for looking up cross-section information: http://atom.kaeri.re.kr/ton/ Go to the nuclide you want, click on cross-section (XS) graphs, select the ENDF, and you can plot any of the categories (MT=whatever) you want. You can also download...
22. ### Why is U235 used for chain reaction but not U238

I think it has more to do with the odd number of neutrons in U-235 than the total number. Atoms with an even number of protons and/or neutrons tend to be more stable than those with odd numbers. A U-239 atom, for example, has about a 40% chance of fissioning if it captures a thermal neutron...
23. ### Need some help with this integral

Thanks arildno. Partial fractions is one of those skills I relearn once a year or so when I need to do a LaPlace Transform and then quickly forget.
24. ### Need some help with this integral

I ran into the following (general form) integral today while working on an engineering problem: \int \frac{dx}{x (a \sqrt{x} +b)} I ran it through 'integrals.com' and came up with the following solution: \frac {\ln{x} - 2 \ln{(a \sqrt{x} + b)}}{b} which is fine for the...
25. ### Can a nuke power plant just blow up?

Morbius: I'm curious. It couldn't be an accident they put one of the best moderating materials known to man on the end of the control rods. Do you have any idea what the intended purpose of the tips was? Some sort of oscillation dampening perhaps?
26. ### Can a nuke power plant just blow up?

A lot of those folks don't do big numbers. I guess anything over 1000 is all the same to them. Too bad the ones protesting Yucca Mountain can handle the large numbers better. :grumpy: One of the saddest 'facts' I ever saw on one of those 'DU is destroying the world' type webpages was this...
27. ### Decay Processes

You can check out the excited states for nuclei here: http://atom.kaeri.re.kr/ton/ In most cases, for the pure beta emitters, the Q value of the reaction is less than the energy of the lowest excited state in the daughter nuclei. For example, N-14's first excited state is at 5.1 Mev, but...
28. ### Can a nuke power plant just blow up?

It took a lot of smart people many years to make the first nuclear bombs. It's not something that just happens. Probably the worst-case scenario for a reactor would be a steam-explosion, like Chernobyl. It was enough to blow chunks of the reactor core through the roof of the reactor building...
29. ### News Registering women for a draft

I think men and women should be treated equally. By that, I mean that neither should have to register for SS. :tongue2: I think it's a dinosaur that has outlived its usefulness, and frankly, isn't compatible with a democratic society. Since that option isn't really on the table, I guess I'd...
30. ### News U.S. Outsourcing and Oil Dealings Begin to Hit Home

Nah, Condi's too high up on the food-chain. They'll pick one of the deputy-undersecretaries from the committee that approved the deal. As far as I know, the WH is sticking with their 'none of our top guys even knew about this' defense.

Nah, I think we're safe until they start getting driver's licenses. I thought my cat was pretty unusual for opening doors, but it sounds like it's pretty common. If I leave a door pushed to, but not latched, one of my cats will reach a paw in and pull the door open. Any of the other three...
32. ### Why do gases absorb neutrons less readily than water?

Carlb: Since that paper is dealing with 'cold' and 'ultra-cold' neutrons, I agree with your assessment of why the cross-sections of liquids and gases are different at those energies. The neutron wavelength is large enough at those energies to interact with multiple atoms rather than just a...
33. ### Information on a fission reaction

What you'll need to do is make some simplifying assumptions. The key component of the 4- and 6- factor formulas are the cross-sections, which represent the probability the neutrons will interact with a given nucleus. Unfortunately, these cross-sections are strongly energy dependent. Take a...

That's pretty impressive actually. I've got a cat who opens doors, drawers, etc, but I've never heard of one that shut the door behind him. I've got 4 myself. I never wanted that many, they just seem to mysteriously multiply. I got the 2nd cat to keep the first one company. Then a girl I...
35. ### Information on a fission reaction

Sure. It's 'Nuclear Reactor Analysis' by Duderstadt and Hamilton. I don't know your level of expertise, but it's a pretty intimidating text (and I say that as a person working on an MS in Nuclear Engineering.) You might also check out 'Introduction to Nuclear Engineering' by Lamarsh. It's a...
36. ### New Flux in a Sphere of moderator of Radius R

Well, like I said, it shouldn't make a difference on this problem. My only concern is that, if it really does make a difference on *some* problems - and I honestly can't say for certain if it does, your instructor might take off a point or two because the equation is technically wrong. I'm...
37. ### New Flux in a Sphere of moderator of Radius R

Arrgh! I just noticed that minus sign in the SINH term. It shouldn't be there. I cut and pasted the latex from the exponential form and forgot to take it out. :blushing: It doesn't change the answer in this case, but in a non-symmetrical problem, I think it would. That general form should...
38. ### Information on a fission reaction

i'm pretty sure these can only be determined emperically. Here's what I've got from my reactor theory book: a) \nu(E) = 2.432 + 0.066E, for 0<E<=1MeV \nu(E) = 2.349 + 0.15E, for E > 1 MeV. \nu is defined as the number of neutrons released per fission, and notice also that \nu is...
39. ### New Flux in a Sphere of moderator of Radius R

It should be. You've got a uniformly distributed source, so if you pick an infinitely small sphere, i.e., a point, you have no source there. At the center of the sphere, you've got the same amount of material -> the same source in every direction, so there should be no net current. If you...
40. ### New Flux in a Sphere of moderator of Radius R

Try this form for a general solution: \phi = A \frac{\sinh{-\frac{r}{L}}}{r} + B \frac{\cosh{\frac{r}{L}}}{r} + C I was taught that if the inhomogeneous part is a polynomial, your solution should be of the same order (0 in this case,) so I agree with the 'C' you posted above. You can...
41. ### New Flux in a Sphere of moderator of Radius R

In the 2nd edition, it's problem 5-16a. I can't speak for the 1st though. Try this: \Sigma @Lucky mkhonza: This problem is going to look a lot like the infinite reactor with a point source. The differences are: 1. You'll have a source term in the diffusion equation, so it will...
42. ### News US Nuclear Deal With India

From http://www.msnbc.msn.com/id/11677308/site/newsweek/ [Broken] article: Not only have they tested nuclear weapons, they haven't signed the NPT, so they're 0-for-2 as far as the law is concerned. Yes, in a 'when we feel like it' way. Under the agreement, their military reactors would...
43. ### News Army to Launch Probe Into Tillman Death

I certainly hope it's a waste of time. That's the best possible outcome in my mind. Even though his statement isn't strictly correct factually, I agree with SA's sentiment on the matter. I don't think a criminal investigation would be happening for your average Joe Private who gets killed by...
44. ### News US Nuclear Deal With India

This deal has been bothering me, and (unfortunately) not getting enough press. It's not a done-deal yet though. The Senate still has to approve it to make it official. I haven't heard any Senators talking about it, which surprises me. It seems like an easy way to score some cheap polictical...
45. ### How is a physics nerd like me supposed to meet women?

I can empathize with you. I went to a Military Academy, so we only had ~20% women to begin with. In our physics department, we had *zero.* I would suggest finding a club/hobby/intramural sport you're interested in that's got at least an even mix of women. As others have pointed out, it...
46. ### News DNA tests disprove Mormon scripture

I did a little searching and this was the closest thing to a 'debunking' I could find: http://www.philipcoppens.com/egyptiancanyon.html It seems to be one of the most 'balanced' articles I could find on the subject. Unfortunately, a lot of the things that pop up when you do a search are...
47. ### News DNA tests disprove Mormon scripture

Your date is off by a few millenia. The BOM says Lehi's exodus took place shortly before the fall of Jerusalem, which has been dated to about 586 BCE. My copy actually states the Exodus took place in 600 BC, but I suspect that date was added by an editor, and not part of the original book...
48. ### News Holocaust denial is a crime

Define injurous. As others have said, if you publish libelous information about other people, you can be sued. If it's something that's likely to bring about physical harm, it's most likely a criminal offense. Sort of like shouting 'fire!' in a crowded theater. As for the original...
49. ### News DNA tests disprove Mormon scripture

When there are verses that specifically refer to white skin and black skin, I don't see how it could be interpreted otherwise. My understanding is that white and black have been changed to pure and impure in later versions of the BoM. I don't think it's a matter of intelligence. I call it...
50. ### Most probable and average value of a function

OK, that does it. Thanks for the help.