Carroll & Ramsey Instruments 101SDC-P User manual

Carroll & Ramsey Instruments 613 Skysail Lane, Fort Collins, CO 80525
Ph. [phone] E-mail: <[e-mail]> <www.cr-instruments.com>
MODEL 101SDC-P SINGLE-CHANNEL
RADIATION DETECTOR
Rev. M2
Copyright June, 2019
The model 101S-DC-P from Carroll & Ramsey Associates is a radiation detector system
intended for radio-chemical synthesis and process monitoring applications in nuclear
medicine laboratories, hot cells, etc.
TECHNICAL DESCRIPTION
The system comprises a miniature detector probe, plus a trans-resistance (current-to-
voltage converter) amplifier.
The maximum gain of the transresistance amplifier is nominally 5 x109ohms, that is, a
detector current of one nano-ampere (negative polarity) can produce a voltage at the
output of the amplifier of +5 volts. A multi-turn trim potentiometer on the output of the
amplifier allows the user to adjust the overall trans-resistance gain of the system over
a range of approximately 20:1.

Probe
The active element in the detector probe is a silicon PIN diode which is enclosed in a
thin brass capsule to shield it against light and stray electromagnetic fields. The sensitive
element is parallel to the face of the probe.
The probe is connected to its amplifier unit through a length of small-diameter coaxial
cable. The detector probe operates in "DC" mode and is intended to be used in close
proximity to concentrated sources of gamma-emitting radio-nuclides.
Three sizes of Si diode probe are available; 7.6 mm2(standard), 25 mm2, and 3 x 30 mm.
The diode substrate thickness is 300 microns, and produces approximately 1 pico-ampere
per mm2per rad / hr (Si). Also available are high-sensitivity scintillation probes with 1
cu cm or 10 cu cm CsI(Tl) crystals coupled to 1 sq cm Si photo-diodes .
The probes are not necessarily symmetrical front-to-back; One face is
somewhat more sensitive than the other – especially when the source of
activity is intimately coupled, as in a flow-cell application. On the smaller
(2.7 x 2.7 mm and 5 x 5 mm probes) the more sensitive face is recognized by
a small ‘bulge’ formed by a solder connection under the shrink-tubing . On
the 3 x 30 mm probe, the more sensitive face is the ‘smooth’ (no-bulge) face
of the probe.
Mechanical
The probes are mechanically robust and are not damaged by normal handling.
However, the probe and its connecting cable may generate spurious signals
("microphonics") if moved or flexed during use. When making a measurement, the probe
and its cable should be held in place and padded if necessary to minimize coupling to
sources of vibration such as fans, pumps, motors, etc.
The amplifier is built into a small aluminum case approximately 6" long x 3" wide x 1.5"
high (exclusive of cable connectors).
DC-Amplifiers
A small positive DC offset -- equivalent to approximately 5 pico-amperes or less (~0.5%
referred to full-scale), in the absence of radiation -- is normally present. Measurements
at very low radiation exposure dose levels are confounded to some degree due to this
offset, and by slow changes in offset -- referred to as "drift" -- which are
indistinguishable from changes in radiation intensity. Other sources of error are detector
diode leakage and "dark" currents--all of which, in turn, are strongly influenced by

ambient temperature. Offset drift is minimized by utilizing high-quality operational
amplifiers with very low initial offset. Leakage current is minimized by operating the
diode at zero bias into a low-impedance ("virtual ground") transresistance amplifier input.
However, the diode's dark current is still of the order of a few pico-amperes at 25oC, and
increases by a factor of 2 for every 5oC rise in temperature. Thus, since the basic probe
sensitivity is of the order of ~1 pA per sq. mm per rad / hr (Si), this detector system is
best suited for applications involving gamma radiation fields of several roentgens per
hour or greater exposure dose-rate, for example, in close proximity to concentrated
sources of gamma activity of 1 mCi or more for the smallest probes, or several 10's of
microcuries (at least) for the larger probes.

Source-Detector distance
See the attached graph to determine
exposure doserate as a function of
distance from a concentrated
("point") source of â+(i.e., 511 KeV
gamma in this example) activity. For
very close coupling to a point source
of activity, the inverse-square law has
a potent effect on detector response.
This can be of some advantage in
reducing the amount of shielding
required against ambient activity
away from the source relative to
activity at the source.
However, if one is monitoring the
amount of activity trapped in a
column, for example, the exact place
within the column where the activity
is trapped can affect the observed
doserate. Moving the detector a short
distance away will mitigate this
effect. The user should experiment
with source-detector placement to
find the best compromise between
repeatability and minimum shielding.
Scale factor
Since observed doserate (and, consequently, detector current) is a function of source-
detector distance, one may place the detector at a particular distance from the source so
as to provide a convenient scale reading. For example, (refer to graph) a 1 curie â+source
produces a field of 200 roentgens / hr at a distance ~5 cm from the source. A "standard"
probe produces ~ 1.5 nanoampere at this exposure doserate, yielding an output signal of
15.0 volts at full gain setting; once the source-detector mounting position is fixed, the
amplifier gain control potentiometer may be adjusted so as to set the scale reading to “10"
to correspond (for example) to “1.0 curie”.

INSTALLATION AND OPERATING ENVIRONMENT
The amplifier box may be placed where radiation is present, but preferably away– or
shielded from – intense sources of radiation.
In North America, the operating voltage for the amplifier box is 110 VAC, 50 / 60
Hz. In Europe the operating voltage is 230 VAC, 50 Hz. A 0.125 ampere “Pico
fuse” is soldered to the circuit board adjacent to the power supply.
Caution! High Voltage
Servicing the fuse should be done only by qualified personnel.
Do not open the amplifier chassis box before unplugging
the system from the AC mains.
The detector probe should be connected to the BNC coaxial receptacle labeled “Probe
in” on the front of the amplifier box. The output signal is taken through a BNC
coaxial receptacle on the rear of the box, adjacent to the power cord. The signal level,
which is proportional to dose-rate, may be monitored by a read-out device such as a
digital voltmeter, high-impedance chart recorder or computer data acquisition system
(1 megohm or more input impedance).
The cable connecting the amplifier to the readout device should be a good-quality
coaxial cable such as RG-174, RG-58AU, or equivalent, and may be any length up to
~100 ft.
The amplifier gain may be adjusted by means of a multi-turn, screwdriver-adjusted
potentiometer which is accessible by removing the top of the amplifier box. Turning
the adjustment screw clockwise increases gain, and vice-versa. The potentiometer is
set at the factory at approximately mid-value. Note: The internal circuitry is sensitive
to ambient light and/or electromagnetic interference, which may cause an excessive
baseline offset when the cover is left off. Gain adjustments should be made in steps,
closing the cover after each increment / decrement until the desired operating point is
found.
Caution! High Voltage
Do not open the amplifier chassis box before unplugging
the system from the AC mains. Do not attempt to adjust
any other components inside the amplifier box.

Environment
The system is intended for indoor use. Components have not been characterized for
operation outside the range 0oC - 55oC. The system components and system wiring
must not be in close proximity to any flame, heating element, or exposed electrical
terminals. The system components have not been characterized for operation in
extreme neutron / gamma radiation fields such as encountered in close proximity to
accelerator targets during irradiation. The system and its components should be
protected from contact with solvents or volatile or corrosive reagents.
EMI susceptibility Nuclear particle accelerators are used for the production of short-
lived radio-isotopes. Such accelerators usually employ high-powered radio-frequency
(RF) systems which have the potential to ‘leak’ RF energy into the environment.
The Model 101-SDC-P radiation detector system is widely -- and successfully -- used
in PET / Radio-chemistry laboratories, which are almost always situated close to a
cyclotron or similar type of ‘RF continuous wave’ nuclear particle accelerator.
However, the system detects and amplifies very low-level input signals. Thus, there is
a possibility that power surges, transient bursts, or RF interference – either radiated or
conducted through power or signal wiring – may occasionally cause spurious or false
outputs. This may occur as a result of fast electrical transients or modulated RF signals --
for example -- from nearby digital cellular telephones or from electrically ‘noisy’ devices
such as on / off (make-and-break) relay contacts, or from small ‘universal’ (AC/DC)
motors often used in small electrical appliances.
For best results, RF devices such as cellular telephones and electrically ‘noisy’
equipment should not be operated in the near vicinity of sensitive radiation detection
equipment (or -- for that matter – near any sensitive electronic instrumentation).

TEST AND MAINTENANCE
Sensitivity
Sensitivity scale factor (numerical display reading) should be verified periodically
against a known standard.
Analog Gain
Amplifier system gain may be verified by connecting a current source comprising a
1.5 VDC battery in series with a high-ohm resistor (1000 megohms or more) to the
amplifier BNC input ( negative polarity)
Probe
The detector probe may be tested in a gross manner as an ordinary diode using a good-
quality ohmmeter with a "diode-test" setting; if it appears that a probe is not working
properly, check for a short- or open-circuit condition. A substantial increase in
reverse leakage current due to radiation damage manifests as an equivalent drop in
reverse resistance, which may, in turn, cause a noticeable increase in observed offset.
However, the reverse resistance of a properly-working detector probe is of the order
of a few hundreds to thousands of megohms, and cannot, in general, be read on a
standard ohm-meter.
When in doubt, compare a suspected probe against another unit which is known to be
sound.

WARRANTY
Systems are warranted against defects in materials and workmanship for a period of 1
year from date of shipment. Carroll & Ramsey's (CRI) sole obligation for products
that prove to be defective will be repair or replacement. In no event shall CRI's
obligation exceed the buyer's purchase price. CRI specifically disclaims any implied
warranties or merchantibility or fitness for a specific purpose, nor will CRI be liable
for any indirect, incidental, or consequential damages.
This warranty does not apply to products which have been subject to mis-use such as
accident, severe mechanical shock and distress, over-voltage, immersion, exposure to
volatile or corrosive agents, etc. The warranty does not apply to defects due to
unauthorized modification, or which have been altered in such a way as to not be
capable of undergoing functional test.
Performance specifications, physical dimensions, etc. are subject to change without
notice.
Products sold by Carroll & Ramsey Instruments are not intended for use as critical
components in medical devices or life-support devices or systems.
(Copyright Carroll & Ramsey Instruments; June, 2019. All Rights Reserved)