stephenbrooks.orgForumMuon1GeneralSome new pages (back at work)
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Stephen Brooks
2002-06-20 16:52:32
I was back at the laboratory this week (and will be for most of the next).  Two URLs may be of interest.

http://stephenbrooks.org/ral/report/
Displays work-in-progress of a report I'm writing on all this.

http://stephenbrooks.org/ral?muon1
Displays the files on a web-based backup I keep of my work HDD.

While writing the report I was thinking about the irritating random behavior (see section 2.2.3) caused by the pion decay and I'm considering various options for changing the simulation to reduce this.  I also noticed than an equation I was using to calculate the motion of the particles simplified algebraically, getting rid of a pesky sqrt() evaluation.  But I tested the speed of that and it is only negligably faster.  There might be a version 4.12 once I've made up my mind what I'm going to change.


"As every 11-year-old kid knows, if you concentrate enough Van-der-Graff generators and expensive special effects in one place, you create a spiral space-time whirly thing, AND an interesting plotline"

[This message was edited by Stephen Brooks on 2002-Jun-21 at 16:05.]

[This message was edited by Stephen Brooks on 2002-Jun-21 at 16:06.]
gogomaus
2002-06-21 13:50:37
Stephen, that´s really great !
Having a look over your shoulder to see actual work/report in progress.
Thanks a lot for your open mind.

I hardly dare to indicate two "mistakes".

One refers to decay time of pions+ to muons+. To my knowledge 26 ns is the value of _mean_ life time = the decay to 1/e and not to 1/2.

The other issue concerns the 3 "clones" of user scottsaxman within your result overview table... wink
Stephen Brooks
2002-06-21 16:04:03
Err you've got me worried temporarily about that 1/e vs. 1/2 thing.  I think all timescales quoted are half-lifes to 1/2 rather than "characteristic" lifetimes to 1/e but I'll re-check with the place I got the data from.

Aha nope - from the very facility where I work comes this page...

http://www.isis.rl.ac.uk/muons/index.htm?content_area=/muons/muonsIntro/isisMuons/isisMuons.htm&side_nav=/muons/muonsSideNav.htm&

...which states the half-life to be 26.0 ns.

Scottsaxman.  I'll look into it.


"As every 11-year-old kid knows, if you concentrate enough Van-der-Graff generators and expensive special effects in one place, you create a spiral space-time whirly thing, AND an interesting plotline"
Stephen Brooks
2002-06-25 15:13:08
Ok that's fixed [bumps this thread to the top again]


"As every 11-year-old kid knows, if you concentrate enough Van-der-Graff generators and expensive special effects in one place, you create a spiral space-time whirly thing, AND an interesting plotline"
Helix_Von_Smelix
2002-06-28 07:09:40
small point i know, but,,,,,,, you may want to ask someone about the information that you have given people access to, with regard to the data protection act.  You could be in deep do do!!!!!  roll eyes roll eyes

Best of luck smile

If you make instant coffee in to a microwave do you go back in time??
gogomaus
2002-07-02 09:28:19
quote:
Originally posted by Stephen Brooks:
Err you've got me worried temporarily about that 1/e vs. 1/2 thing.  I think all timescales quoted are half-lifes to 1/2 rather than "characteristic" lifetimes to 1/e but I'll re-check with the place I got the data from.

Aha nope - from the very facility where I work comes this page...

http://www.isis.rl.ac.uk/muons/index.htm?content_area=/muons/muonsIntro/isisMuons/isisMuons.htm&side_nav=/muons/muonsSideNav.htm&

...which states the half-life to be 26.0 ns.


"As every 11-year-old kid knows, if you concentrate enough Van-der-Graff generators and expensive special effects in one place, you create a _spiral space-time whirly thing_, AND an interesting plotline"


Hello again,
meanwhile I had a look on above cited sites.  They are pretty nice PR-educational pages, but I´m still concerned about data quality.  F.e. muons are reported with a life time of 2.2 µs; that´s in fact their correct mean life time, and not a half-life !!!
(Your report also uses 1.5x µs for half-life of muons !).

I got my info from the particle physics booklet (Rev.  of Part.Physics; D.E.Groom et al., The European Physics Journal C15 (2000)).
Data are available under http://pdg.lbl.gov -> particle listings 2001 -> mesons -> pi+ions = ( pdg.lbl.gov/2001/s008.pdf ).
Find at bottom of page 2 value for _mean_ life time = 26.033 +/-0.005 ns .

The meaning of _mean_ life time is defined as the time for reducing initial amount of particles to 1/e ; while half-life is used for decay down to 1/2.

Many people tend to mix up both meanings/values.  I found a noteworthy example on a website not to far away from you, the Durham/RAL database, which is linked under pdg.lbl.gov -> Part.Physics Info+databases.  Look for SLAC/VVC under the HEPDATA screen; then use theory and search for "life-time and half-life"
( http://cpt19.dur.ac.uk/vcc/theory/halflife.html ).

You will find a graph and the well-known equation for statistically determinated decay.  A mean life time T is defined and also a half-life t1/2; simple mathematical operations lead to the final conclusion : t1/2 = ln(2)*T.
That´s fine and absolutely correct.
Unfortunately the graph N(t) shows No/2 for time = T on x-axis.  (Thus would be a prove that ln(2) = 1; hahaha...).

Maybe, you now understand my scepticism about correct data on public web-sites.
It´s really a mess using decay time values with so much laissez-faire.

Please, try to use a real reliable data source, otherwise all your simulations have to be revised (the later the worse).
Hope, you will accept the inconvenience caused by me ; but isn´t it better to do it now instead of creating a lot of work for a cucumber castle ?  eek ?
Stephen Brooks
2002-07-07 05:28:14
I think the error here has appeared from a mis-naming of a variable in my simulation.  Let me explain -

I found the original paper where I got the lifetime data from (it was not a PR site - I just used a random other site to do a quick check that my figure was correct, and apparently that site also made exactly the same error, which is confusing), and it does indeed say "mean life" rather than half-life.

Then in my source-code I had

#define Thalf_pi 2.6033e-8

...so it looked for a while as if the simulations were wrong.  Then I checked where I worked out the actual decay times, and my disribution was calculated from

pn->t_decay=-log(frnd(1))*Thalf_pi;

...which is the correct formula if Thalf_pi was the MEAN lifetime, which by mistake it was.  So the only correction to my code I had to make was renaming Thalf_pi to T_pi and adding a comment about this being the mean-life not the half-life.

I honestly don't know whether this resulted from two consecutive mistakes that cancelled, or whether I really did know what I was doing all along but used a non-intuitive constant name in my code!

However the value in my report _is_ incorrect, so I shall now change that.


"As every 11-year-old kid knows, if you concentrate enough Van-der-Graff generators and expensive special effects in one place, you create a spiral space-time whirly thing, AND an interesting plotline"
Stephen Brooks
2002-07-07 06:27:44
quote:
Originally posted by Helix_Von_Smelix:
small point i know, but,,,,,,, you may want to ask someone about the information that you have given people access to, with regard to the data protection act.  You could be in deep do do!!!!!


Don't worry - my site already violates the data protection act, so putting that up is no big change.  Besides, there's not actaully _that_ much sensitive information in there (if any).


"As every 11-year-old kid knows, if you concentrate enough Van-der-Graff generators and expensive special effects in one place, you create a spiral space-time whirly thing, AND an interesting plotline"
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