Neutron Source Anisotropy

[tdc@ehssun.lbl.gov]

This posting should be of interest to those of you using PuBe neutron
sources for neutron calibrations - especially the old Mound sources.

For the past few months I have been measuring the anisotropy of 14 neutron
sources from 10^4 to almost 10^8 4 pi neutrons/sec emission.  Some 238
some 239, 2 AmBe.  MOST PuBe but one each Pu238 Li, F and B.  A few were
looked at rotating about 2 orthogonal axes and a couple will be looked at
in a pipe nipple (2R) container.  Counts are taken every 15 degrees and
both Flux and Dose moderated detectors are used.  Counting times are
10,000 or 20,000 seconds for good statistics (except for the 10^4 source -
I wasn't willing to count that one as long as necessary!) so each run
takes 3 or 6 days.

This work is getting close to done - maybe only a month or so left.

This assessment is necessary since NIST will only calibrate neutron
sources for 4 Pi emission.  Simple calculations for flux or dose at a
distance could be considerably off if this anisotropy is not taken into
consideration!  Also if you calculate scatter using the Jenkin's Formula
- knowing the anisotropy (or orienting the source so a separate correction
is unnecessary) can be helpful.

As this effort has been progressing I have been putting the results on an
anonymous FTP site in two forms: A Quattro Pro WB1 spreadsheet and a
POSTSCRIPT output file of the printout of this spreadsheet to be copied to
any PostScript printer.

For those interested and who understand FTP - the site is ehssun1.lbl.gov
and the directory is /tdcpub/isotropy.

For those who only access the internet via browsers and may not be
familiar with FTP - it can work just like HTTP!  Just use the "document
line" -

ftp://ehssun1.lbl.gov/tdcpub/isotropy

You will see a directory listing of files named by in-house source number.
Download whatever you like.

For Mound sources - It is my understanding that they where built in a few
standard aspect ratios (physical sizes) and so if your source is close to
mine then the anisotropy should be about the same.

IF you have 239 PuBe about 10^5  see source 551

IF you have 239 PuBe about 1.5x10^6  see source 571, 581 or 582

IF you have 239 PuBe about 7x10^6  see source 591, 592 or 593

IF you have 238 PuBe about 2x10^4  see source 631

IF you have 238 PuBe about 7x10^7  see source 632

IF you have 238 PuLi about 3x10^6  see source 633

IF you have 238 PuB about 7x10^6  see source 634

IF you have 238 PuF about 4x10^6  see source 651

IF you have AmBe about 2x10^5  see either NE source

Most file names have an "H" in them.  This means the source was laying on
its side Horizontally.  A "V" means vertical and a "B" means its in a pipe
nipple secondary container ..... don't ask!

Hopefully this may be of interest to someone else too!  For us this has
also been an opportunity to calibrate older sources in reference to ones
just back from NIST calibration.