Effect of lead and HDPE on fast neutron flux

[Jon Rosenstiel]

A quick test to see the effect of lead and HDPE on the fusor’s fast neutron flux. Detector element is a 2” diameter by 5/8” thick BC-720 replica fast neutron detector. Note that these types of detectors are insensitive to fast neutrons below 1 MeV.

Flux with no absorber: 20.4 cps
Flux with 2” of lead: 14.7 cps (-28%)
Flux with 2” of HDPE: 8.8 cps (-57%)

Jon Rosenstiel

No absorber

2-inches of lead

2-inches of HDPE

[Richard Hull]

Thanks Jon for that effort. Lead has an incredibly tiny cross section thus there is no real activation due to fast or thermal neutrons, but there is, due to its sheer density, scattering and some reflection. HDPE due to hydrogen is both slowing and scattering and, if large enough volume, absorbing. I will not be using significant lead shielding with my HDPE moderator. I might have to build my 5 inch BC-720 fast neutron detector with one of my 5" hamamatsu PMT's. My digital NIM counter will enjoy the respite from the rush of 3He counts. Then the 3He moderator will just become my neutron oven.

Richard Hull

[Jon Rosenstiel]

Richard,

Yeah, I was reading about your experience with the 5/8" lead shielding and figured this would be a good time to put this up on the forum. As we know, the BC-720 is pretty-much worthless for the beginner trying to see if his (or her) brand new fusor is producing neutrons, just not sensitive enough, lots of electrical noise, etc., etc. You need to mate that 5" BC-720 to a pmt, I know you'll be pleased with the results.

JonR

[Richard Hull]

You are right, Jon. I have the time, I have the parts, and covid has moderately trapped me and countless millions to not go out and have the blast of happier times. I have been busy on both fusor V efforts, moderator construction and on instant activation of fast isotopes, fighting noise issues, (of my own making), designing new GM counter systems, etc.

It is perhaps a sign that I was overpaid at my old job, when I could spend $400 plus on the 5" BC-720 in 2001 and never use it. I did use my 2-inch, BC720, as you are now using yours, and it worked, but as you note, was not a performer on fusor III. At the time I was plugged into Don Orie and his vast empire at O.E. technologies. He suggested I get one of his large BF3 tubes. I did, but before I could even test it, Don fell into that stock of 3He tubes and I bought three of the 22" inch P4 units. Other than the 2" BC-720, I have never used or relied heavily on any neutron counting system more than the 3He tube. (spoiled rotten)

There was a small slot of time during the 3 years of fusor III that I used the classic Eberline PNC-1 in conjunction with the Eberline PNR-4 rem ball system and the Large black HDPE moderated "Snoopy" which were a tiny bit better than the BC-720. I was very happy with the classic GE boron lined tube, once I got the devilish thing hyper fine tuned to not detect anything but neutrons. However, the work with the 3He tube just blew everything away for low end neutron detection and was a snap to make work with virtually zero effort at the electronics end of things.

I will now, among many other things, endeavor to get busy on the big BC-720 assembly. The BC-720 is definitely not for the newbie just trying to squeeze out those first neutrons. Weak sources need not apply to the BC-720 for results. I am hoping the 5" with its larger volume and area can shine much more brightly than the 2"....After all, it is why I bought it 18 years ago!

A couple of old images with old detectors for the general interest of those when I went down the "rabbit hole". As always, click on image to enlarge & use back arrow to come back

Richard Hull