Beta autoradiography forms a standard procedure for studying metabolic processes in
biological systems. Beta ray emitting tracers, namely like H-3, C-14, P-32 and S-35 are
attached with chemical methods to biological molecules. Hereafter, the molecules undergo,
in vivo or in vitro, metabolic processes. The nature of those processes may be analyzed
by i) identifying product molecules by spotting the radio tracers and/or ii) observing
the position of beta emissions related to the radio tracers from tissue probes (e.g. slices of rat brain).
While the bio-chemical side of autoradiographie is well established, the radiation detection
relies on elder technologies like photographic films, phosphor-plates and scintillator
plates, which are typically exposed for several 10 hours to the radiation of the sample.
Due to the limited sensitivity and contrast of those systems, the results of those
slow measurements show frequently saturated regions while weaker sources are not identified.
Moreover, the spatial resolution of the measurements is limited as the beta rays
are emitted into 4P and may impinge the sensor with a shallow angle and rather distant
from the true emission point.
CMOS Monolithic Active Pixel Sensors (MAPS) as being employed for present and future
experiments in particle and heavy ion physics (e.g. STAR, NA61, CBM, ALICE) may overcome
this limitations. The devices are suited to detect beta rays with excellent detection efficiency,
very high rate capability and outstanding spatial resolution. Moreover, back-thinned MAPS
were shown to be sensitive even to the ~10 keV beta rays of H-3, which are absorbed
by the entrance window of almost any other detector system. Finally, MAPS are mostly
transparent to natural gamma radiation, which helps to reduce the background of
In a first pilot study, we tested the capability of MAPS to recognize RNA fragments,
which were marked with P-32 and separated by electrophoresis in a next step. The
sample used was produced for regular research and our experiment was carried out
once it became obsolete. This and the short half-life of P32 turned into a substantial
reduction of the activity of the sample. The MIMOSA-26AHR sensor employed for
the test was exposed to the radiation of sources of < few Bq (detection limit of an independent
measurement with a Contamat) for several hours. Despite no hardware and few
software optimization was carried out, radiation peaks caused by the RNA-fragments were
indicated in front of a very low background.
Based on this positive result and our experience with the integration of MAPS
into sizable detector systems, we propose to explore the feasibility of employing those
sensors in detection systems for beta-autoradiography and to provide on the long term those detection systems to the community. It is anticipated that this improved system will provide multiple advantages with respect to the established detector systems. Among others, we expect:
A simplified quantitative analysis of the signal as a direct hit counting is carried out.
An strongly improved contrast (in theory >> 1:1e6), which will allow for identifying
weak radiation sources without tolerating saturation effects in other parts of the picture.
An advanced signal over background ration of the measurements, which is provided by
the excellent sensitivity of MAPS (>99,9% of MIPS, to be confirmed for soft beta rays) and the
outstandingly low background observed in our pioneering study.
It is expected that this sensitivity will allow to accelerate regular measurements.
More importantly, one might consider to reduce the amount of radio tracers needed for
a standard test. If successful, this will turn into significant improvements
with respect to work security in the related radio chemistry laboratories and into a reduction
of the nuclear waste related to the test.
An improvement of the spatial resolution of the pictures. This is possible as one obtains
access to the individual beta hits. Based on a cluster shape analyses, it is conceptually possible to
identify and reject beta rays impinging the sensors with a high angle. This reduces the smearing
of the picture caused by beta rays, which are emitted mostly parallel to the sensor surface.
The option to generate digital high resolution pictures with radio traces with small beta energy
Beta autoradiography is an established tool for analyzing biochemical processes in life science. It consists in marking bio-molekules, which are expected to undergo a metabolic process, with radioactive markers and to identify the location or nature of the reaction products by identifying them based on the related beta emission.
It is proposed to improve this methode by replacing the traditional radiation sensors (e.g. photographic films, phosphor plates) by CMOS Monolithic Active Pixel Sensors, which were developed for vertex detectors of experiments in particle and heavy ion physics. Besides improving the sensitivity of the method, this may reduce the amount of required radio tracers. If so, one will presumabyl create less nuclear waste and improve the work security in the related radio-chemical laboratories.