RRFM 2009 Transactions - European Nuclear Society

SILICON DOPING AT FRM II
H. GERSTENBERG, X. LI, I. NEUHAUS
TU München, Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II)
Lichtenbergstrasse 1, D-85747 Garching - Germany
ABSTRACT
FRM II, the 20 MW heavy water moderated research reactor of the Technische
Universität München, is equipped with a Si doping facility suitable for the irradiation
of ingots up to a maximum diameter of 200 mm and a maximum length of 500 mm.
Using the results of test experiments and additional MCNP calculations which had
already been performed during the nuclear commissioning of FRM II a Ni absorber
surrounding the irradiation position had been shaped suitably to achieve the
necessary high homogeneity of the doping profile. The installation of the irradiation
channel within the heavy water moderator tank providing a very well thermalized
neutron spectrum makes the facility particularly interesting for the doping aiming
high target resistivities up to > 1000 Ωcm. Since 2007 the facility is working
semiautomatically. The high demand from the industry initiated the implementation
of a 2-shift operation mode which allowed the irradiation of more about 10 t of Si in
2008.
1. Introduction
The Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II) is a 20 MW heavy water
moderated research reactor being operated since 2005 by the Technische Universität
München on its campus in Garching. Due to its very compact core design – only a single
cylindrical fuel element containing approximately 8 kg of highly enriched uranium – the
reactor is clearly optimized for basic research by means of neutron beam tube experiments.
Nonetheless it was evident already since the very beginning of the reactor project that FRM II
has also to be made available to medical, industrial and commercial applications. For this
purpose the reactor was equipped in addition to the scientific installations with a facility for
cancer treatment by neutron irradiation [1] and several irradiation channels for technical
applications [2]. Among those the Si doping facility turned out to be the most important and
successful one from the commercial point of view.
2. Basic layout of the Si doping facility
The neutron transmutation doping (NTD) of the semiconductor Si in a research reactor is
based on the nuclear reaction
30
31 β 31
Si( n,
γ ) Si⎯⎯→
−
P
It is important to note that no further neutron capture reactions take place in Si and that the
half life of 31 Si is only 2.62 h.
The main advantage of NTD as compared to other Si doping techniques is the high
achievable accuracy and homogeneity of the doping profile. For commercial purposes the
resistivity – the most common measure for the doping effect – has to meet the target
resistivity to within ±5 %, and the inhomogeneity of the resistivity along the axis of the ingot
has to be less than again ±5 %. NTD Si fulfilling these requirements is used by all major
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