New irradiation of DMILL bipolar transistors was done in reactor in Ljubljana. The
aim of experiment was to check the influence of thermal neutrons on beta degradation
in DMILL npn bipolar transistors.
Main motivation for this was to understand larger damage observed after irradiation in
reactor in Ljubljana compared to irradiation to the same NIEL fluence in other sources (PS, Prospero ....) . This effect was observed in DMILL bipolar transistors, ABCD3T chips and
also in TRT ASICs (which is also DMILL).

One hyphothesis which could explain this effect was about thermal neutrons.
There are plenty of them in Ljubljana reactor and not so many in other sources.
One mechanism could be neutron capture by boron:  B+n = Li + alpha + 2.3 MeV of energy which could  contribute to NIEL. Since B is used as dopant in DMILL transistors this could have some effect.

To check the influence of thermal neutrons the following experiment was done:

1. Irradiate a sample in "normal way" in our small irradiation tube to 2x10^14 n/cm2
    this means that the sample will receive about 2x10^14 n/cm2 of fast neutrons
    and 4x10^14 n/cm2 of thermal neutrons
    (more in /u/cplear/mandic/public_html/TestStruct/Struct.html )

2. Irradiate another sample in the same tube to the same fluence but now inside a
    container made of ~ 0.5 mm thick Cd. This should absorb almost all thermal neutrons
    and a maximum of few % of fast neutrons.

In the plot  BetaBefore_1.eps beta vs. collector current is drawn for 6 DMILL transistors of different sizes from 2 test structures before irradiation.
In the plot  BetaAfter_1.eps beta after irradiation is shown. The blue lines are for irradiation
inside Cd container and red  for "normal irradiation". Although the measurements of
beta are not very good at higher Ic it  is obvious that damage inside container is smaller than without it.

The results of this irradiation are also consistent with previous DMILL test structures irradiations (see  /u/cplear/mandic/public_html/TestStruct/Struct.html ). It is also consistent
with the difference observed after irradiation in the big and small tube (see
 http://www-f9.ijs.si/~mandic/TestStruct/Beta-1At100.eps , and
http://www-f9.ijs.si/~mandic/TestStruct/BetaIc.eps ) since the fluence of thermal neutrons
after irradiation in the big tube is about a factor of 2 smaller than in the small tube.