Thermo Scientific RO-20 User manual

Operating Manual RF-000-xxxxxx
RO-20 AA Ion Chamber

FHT RF- 0-1
REVISIONS SHEET:
Rev. Rev. Dept. Name Rev. Cat. Explanation
State resp. page *)
*) Category C: editorial correction
I: clearing improvement
A: substantial amendment
Explanations must be given, at least with Category A.

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CONTENTS:
1. Introduction..................................................................................1-1
1.1 Instrument Description ..........................................................................................................1-1
1.2 Measuring Units..................................................................................................................... 1-2
1.3 Detector Specifications .......................................................................................................... 1-2
1.3.1 Size ................................................................................................................................. 1-2
1.3.2 Fill................................................................................................................................... 1-2
1.3.3 Wall ................................................................................................................................ 1-2
1.3.4 Window ..........................................................................................................................1-2
1.3.5 Beta Shield...................................................................................................................... 1-2
1.3.6 Radiation Detected ......................................................................................................... 1-2
1.3.7 Photon Energy Response ................................................................................................ 1-2
1.3.8 Examples of Beta Response ........................................................................................... 1-3
1.3.9 Fast Neutron Response (PuBe)....................................................................................... 1-3
1.4 General Specifications ........................................................................................................... 1-4
1.4.1 Ranges Five linear ranges.............................................................................................. 1-4
1.4.2 Meter............................................................................................................................... 1-4
1.4.3 Response Time ............................................................................................................... 1-4
1.4.4 Linearity ......................................................................................................................... 1-4
1.4.5 Controls .......................................................................................................................... 1-4
1.4.5.1 External ................................................................................................................... 1-4
1.4.5.2 Internal .................................................................................................................... 1-5
1.4.6 Batteries .......................................................................................................................... 1-5
1.4.7 Environment ................................................................................................................... 1-5
1.4.7.1 Temperature ............................................................................................................ 1-5
1.4.7.2 Temperature Compensation .................................................................................... 1-5
1.4.7.3 Moisture .................................................................................................................. 1-5
1.4.7.4 Humidity.................................................................................................................. 1-5
1.4.7.5 Weight ..................................................................................................................... 1-6
1.4.7.6 Size .......................................................................................................................... 1-6
2. Operation......................................................................................2-1
2.1 Description of Controls.......................................................................................................... 2-1
2.1.1 Function Switch.............................................................................................................. 2-1
2.1.2 Zero Knob.......................................................................................................................2-1
2.1.3 Light Switch ................................................................................................................... 2-1
2.1.4 Calibration Controls (Internal) ....................................................................................... 2-1
2.2 Using the Instrument.............................................................................................................. 2-1

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2.3 Operation the Instrument ....................................................................................................... 2-2
2.3.1 Checking the Zero Setting .............................................................................................. 2-2
2.3.2 Minimizing Deflection ................................................................................................... 2-2
2.3.3 Ion Chamber ................................................................................................................... 2-2
2.3.4 Radiation Overexposure ................................................................................................. 2-2
2.3.5 Electronic Circuitry ........................................................................................................ 2-2
3. Theory of Operation ....................................................................3-1
3.1 General Theory ...................................................................................................................... 3-1
3.2 Functional Theory.................................................................................................................. 3-2
3.2.1 Ion Chamber ................................................................................................................... 3-2
3.2.2 The Circuit...................................................................................................................... 3-3
4. Maintenance.................................................................................4-1
4.1 Preventive Maintenance......................................................................................................... 4-1
4.1.1 Batteries .......................................................................................................................... 4-1
4.1.2 Desiccant ........................................................................................................................ 4-2
4.1.3 Calibration ...................................................................................................................... 4-2
4.1.4 Circuit Checks ................................................................................................................ 4-3
4.1.5 Disassembly.................................................................................................................... 4-3
4.1.6 Reassembly..................................................................................................................... 4-5
4.1.7 Troubleshooting.............................................................................................................. 4-7
5. Appendix A - Electronics and Parts List .................................5-1

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TABLE OF FIGURES:
Figure 1-1: Model RO-20 Ion Chamber............................................................................................... 1-1
Figure 1-2: Nominal photon energy response ......................................................................................1-3
Figure 3-1: Connector / battery board component layout .................................................................... 3-1
Figure 3-2: Basic operation of the circuit............................................................................................. 3-2
Figure 3-3: Detector board component layout...................................................................................... 3-3
Figure 3-4: Main circuit board layout .................................................................................................. 3-4
Figure 4-1: Interior View, RO-20......................................................................................................... 4-1
Figure 4-2: General schematic ............................................................................................................. 4-9
Table 2-1: Altitude Corrections in Feet (1) ............................................................................................ 2-3
Table 2-2: Altitude Corrections in Meters (2) ........................................................................................ 2-4
Table 5-1: RO-20 Electronics and Parts............................................................................................... 5-1
WEEE Compliance:
This product is required to comply with the European Union’s Waste Electrical & Electronic Equip-
ment (WEEE) Directive 2002/96/EC. It is marked with the following symbol:
Thermo Electron has contracted with one or more recycling/disposal companies in each EU Member
State, and this product should be disposed of or recycled through them. Further information on
Thermo Electron’s compliance with these Directives, the recyclers in your country, and information
on Thermo Electron products which may assist the detection of substances subject to the RoHS
Directive are available at www.thermo.com/WEEERoHS

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1. Introduction
1.1 Instrument Description
The Model RO-20 Ion Chamber (Figure 1-1) is a portable air ion chamber instrument used to
detect beta ( ß ), gamma (γ) and x-ray radiation.
The RO-20 has five linear ranges of operation to measure exposure rate for x-ray and gamma
radiation. The ion chamber is vented to atmospheric pressure.
The RO-20 is calibrated to γradiation (137Cs).
A single rotary Function Switch turns the instrument off, provides battery checks, checks the
Zero setting, and selects the range of operation.
Figure 1-1: Model RO-20 Ion Chamber

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1.2 Measuring Units
The RO-20 has the ranges labeled in exposure rate R/h.
The ranges are 50 R/h,5 R/h and 500mR/h,50mR/h,5 mR/h.
The RO-20 chamber is designed to have a flat energy response in roentgen units.
Use the procedure given in this manual for calibration of the instrument. The calibration
controls on the main circuit board are marked from 5 through 50 k, which corresponds to the
5 ranges (expressed in R/h).
1.3 Detector Specifications
1.3.1 Size
2.84-inch inside diam, 13.4 in3volume (7.21 cm diam, 220 cm3).
1.3.2 Fill
Air, vented to atmospheric pressure.
1.3.3 Wall
0.20 in. (0.51 cm) conductive plastic approximately 640 mg/cm2inside
0.063-in. wall aluminum case, making approximately 1000 mg/cm2total thickness.
1.3.4 Window
Two layers (one on the chamber, one on the can) 0.001-in. (25 micron) mylar, approximately
7 mg/cm2total.
1.3.5 Beta Shield
Sliding shield on bottom of case with positive friction lock. Approximately 1000 mg/cm2
(5/16 in. phenolic).
1.3.6 Radiation Detected
Beta, gamma and x-ray.
1.3.7 Photon Energy Response
The nominal photon energy response is shown in Figure 1-2.

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In the following examples, the RO-20 had been calibrated to 137Cs (slide closed), with the
slide facing the source.
•±30% from 8 keV to 1.3 MeV, with the open slide facing the source.
•±15% from 33 keV to 1.3 MeV, with the closed slide facing the source.
•±15% from 55 keV to 1.3 MeV, through the side of the instrument.
1.3.8 Examples of Beta Response
•Uranium slab: 30% of true mrad/h field behind 7 mg/cm2window, with the RO-20
resting on the slab (slide open).
•90Sr–90Y: Approximately 93% of true mrad/h field at 30 cm with slide open.
1.3.9 Fast Neutron Response (PuBe)
Reads approximately 8% in mR/h of true neutron field in mrem/h.
Figure 1-2: Nominal photon energy response

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1.4 General Specifications
1.4.1 Ranges Five linear ranges
•0–5 mR/h
•0–50 mR/h
•0–500 mR/h
•0–5 R/h
•0–50 R/h
1.4.2 Meter
•Scale length: Approx. 3 in. (7.6 cm)
•2% accuracy.
•Linear markings: From 0 to 5 in 50 minor increments.
•Back-lighted.
1.4.3 Response Time
Five seconds, 0 to 90% of final reading.
1.4.4 Linearity
•Within ± 5% of full scale
•Battery Dependence (Within ± 5%)
•The reading is independent of battery voltage when the battery check indications are in
the green arc. In order to achieve, but linearity in the 50R/h Range, it is recommended
to exchange the 3 ea 12 Volt batteries (indicated by “Battery 2”) once the needle falls
below “4.0”.
1.4.5 Controls
1.4.5.1 External
•Function Switch with Off, Zero, and Battery checking positions.
•The Zero Knob is used to set the meter to zero when the Zero position of the Function
Switch is selected, or when the detector is not in a significant radiation field.

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•Light Switch, for meter light.
1.4.5.2 Internal
•Five calibration controls, one for each range.
1.4.6 Batteries
•Main power: Five AA cells.
•Chamber bias: 3 ea 12V alkaline batteries = 36V.
•Life: AA cells, widely variable according to RO-20 usage and battery type.
•Typical alkaline: mR/h ranges, 2500 hrs; all other positions, 130 hrs.
•36 Volt chamber bias battery life: Totally dependent upon the usage of the “Battery 2”
position of the range switch. The battery capacity should allow for at least 50,000 five-
second battery checks. The battery drain is negligible on all other positions of the
Function Switch.
Note : Frequent or continuous use of the light will reduce battery life significantly. ▲
1.4.7 Environment
1.4.7.1 Temperature
Operable from –40 °F to 140 °F (–40 °C to 60 °C). For operation below 0°F (–18 °C),
alkaline or nickel-metal-hydride AA cells should be used.
1.4.7.2 Temperature Compensation
The detector is fully compensated over the operational temperature range, for output accuracy
within 10% ± 0.5 mR/h.
1.4.7.3 Moisture
Seals used at openings for dust and water resistance. Detector is protected by a silica-gel
desiccant.
1.4.7.4 Humidity
Operable from 0 to 95%, no condensing.

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1.4.7.5 Weight
Approximately 3.6 pounds (1.63 kg) with alkaline AA cells.
1.4.7.6 Size
4.2 in. wide x7.9 in. long x7.7 in. high (10.7 cm x20.1 cm x19.6 cm), including handle.

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2. Operation
2.1 Description of Controls
2.1.1 Function Switch
The function switch is a nine-position rotary switch that turns the instrument off, checks the
condition of the batteries, checks instrument zero, and selects the range of operation to be
used.
2.1.2 Zero Knob
The Zero Knob is used to set the meter to zero when the Zero position of the Function Switch
is selected or when the detector is not in a significant radiation field (< 30µR/h).
2.1.3 Light Switch
The Light Switch illuminates the meter. It has three positions: Off, Momentary and On.
2.1.4 Calibration Controls (Internal)
Five variable resistors, one for each range, provide internal calibration controls.
2.2 Using the Instrument
The higher radiation field strengths measurable by the RO-20 can cause personal injury in a
short time.
Before using the instrument check Battery 1 (AA cells) and Battery 2 (l2V alkaline cells). The
meter should indicate in the green Battery Check arc. Do not leave the Function Switch in the
Battery 2 position for an extended period.
Turn the Function Switch to the Zero position. Check that the meter reads “0.” If not, set it to
‘0’ with the Zero Knob.
Set the Function switch to the desired range of operation. The switch position selected is the
full-scale reading of that range.
When measuring ß, low-energy γor x-ray emissions, open the sliding ß shield on the bottom
of the case and face the bottom of the instrument toward the radiation source. To open or
close the shield, press the friction-release button on the left side of the case and manually
move the slide or let it fall due to gravity. When the shield is open, protect the thin face
against puncture damage.

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2.3 Operation the Instrument
2.3.1 Checking the Zero Setting
The zero setting of the RO-20 may be checked in any radiation field by selecting the Zero position of
the Function Switch.
If a minor offset from zero exists on the most sensitive range when in a known insignificant field, the
Zero Knob may be used to remove the offset. Zero need not be reset when using the higher ranges.
2.3.2 Minimizing Deflection
When switching from the R/h ranges to the mR/h ranges, transient noise may cause a temporary de-
flection of the meter. The deflection can be minimized by first selecting 500 mR/h, letting the needle
settle, and then switching to the lower ranges.
2.3.3 Ion Chamber
The effective center of the ion chamber is marked by dimples at the front and sides of the RO-20.
Because the ion chamber is vented to the atmosphere, it is sensitive to changes in atmospheric pres-
sure. Table 2-1 and Table 2-2 give correction factors to be applied if the RO-20 is used at an elevation
other than the elevation at which it was calibrated. Normal atmospheric pressure variations at one lo-
cation are small enough to be ignored.
The chamber output signal is fully temperature compensated. No additional temperature correction is
necessary.
2.3.4 Radiation Overexposure
The RO-20 will not be damaged by exposing it to radiation fields that exceed the full-scale level. Ex-
treme overexposure may cause a minor shift of the Zero position and a small temporary offset on the
most sensitive range. Resetting the Zero Knob may be necessary.
2.3.5 Electronic Circuitry
The chamber bias battery holds the electronic circuitry at 36 volts negative with respect to chassis
potential. Do not short any part of the circuitry to the chassis parts or damage may occur.

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Table 2-1: Altitude Corrections in Feet (1)
Altitude When Calibrated
Sea level (ft) 1000' 2000' 3000' 4000' 5000' 6000' 7000' 8000' 9000' 10,000'
Sea level (ft) 1 0.96 0.93 0.90 0.86 0.83 0.80 0.77 0.74 0.71 0.69
1000' 1.04 1 0.96 0.93 0.90 0.86 0.83 0.80 0.77 0.74 0.71
2000' 1.08 1.04 1 0.96 0.93 0.89 0.86 0.83 0.80 0.77 0.74
3000' 1.12 1.08 1.04 1 0.96 0.93 0.89 0.86 0.83 0.80 0.77
4000' 1.16 1.12 1.08 1.04 1 0.96 0.93 0.89 0.86 0.83 0.80
5000' 1.20 1.16 1.12 1.08 1.04 1 0.96 0.93 0.89 0.86 0.83
6000' 1.25 1.20 1.16 1.12 1.08 1.04 1 0.96 0.93 0.89 0.86
7000' 1.30 1.25 1.20 1.16 1.12 1.08 1.04 1 0.96 0.93 0.89
8000' 1.35 1.30 1.25 1.21 1.16 1.12 1.08 1.04 1 0.96 0.93
9000' 1.40 1.35 1.30 1.25 1.21 1.16 1.12 1.08 1.04 1 0.96
Altitude when used
10,000' 1.45 1.40 1.35 1.30 1.26 1.21 1.17 1.12 1.08 1.04 1
(1) Multiply reading in feet by correction factor shown.

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Table 2-2: Altitude Corrections in Meters (2)
Altitude When Calibrated
Sea level (m) 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000 3250
Sea level (m) 1 0.97 0.94 0.91 0.89 0.86 0.83 0.81 0.78 0.76 0.73 0.72 0.69 0.67
250 1.03 1 0.97 0.94 0.91 0.89 0.86 0.83 0.81 0.78 0.76 0.74 0.72 0.69
500 1.06 1.03 1 0.97 0.94 0.91 0.89 0.86 0.83 0.81 0.78 0.76 0.74 0.72
750 1.10 1.06 1.03 1 0.97 0.94 0.91 0.89 0.86 0.83 0.81 0.78 0.76 0.74
1000 1.13 1.10 1.06 1.03 1 0.97 0.94 0.91 0.89 0.86 0.83 0.81 0.78 0.76
1250 1.16 1.13 1.10 1.06 1.03 1 0.97 0.94 0.91 0.89 0.86 0.83 0.81 0.78
1500 1.20 1.16 1.13 1.10 1.06 1.03 1 0.97 0.94 0.91 0.89 0.86 0.83 0.81
1750 1.24 1.20 1.16 1.13 1.10 1.06 1.03 1 0.97 0.94 0.91 0.89 0.86 0.83
2000 1.28 1.24 1.20 1.16 1.13 1.10 1.06 1.03 1 0.97 0.94 0.91 0.89 0.86
2250 1.32 1.28 1.24 1.20 1.16 1.13 1.10 1.06 1.03 1 0.97 0.94 0.91 0.89
2500 1.36 1.32 1.28 1.24 1.20 1.16 1.13 1.10 1.06 1.03 1 0.97 0.94 0.91
2750 1.40 1.36 1.32 1.28 1.24 1.20 1.16 1.13 1.10 1.06 1.03 1 0.97 0.94
3000 1.44 1.40 1.36 1.32 1.28 1.24 1.20 1.16 1.13 1.10 1.06 1.03 1 0.97
Altitude when used
3250 1.49 1.44 1.40 1.36 1.32 1.28 1.24 1.20 1.16 1.13 1.10 1.06 1.03 1
(2) Multiply reading in meters by correction factor shown.

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3. Theory of Operation
3.1 General Theory
Figure 3-1 and Figure 3-2 show the basic operation of the circuit. The ion chamber structure is
maintained at chassis potential while all the other circuitry is held at –36 volts by the 3 ea 12-
volt bias batteries. When the air in the chamber ionizes due to radiation, a minute current
flows from the chamber wall to the center electrode, causing the minus input lead of the first
operational amplifier to go very slightly positive. This change results in a negative swing of
the amplifier output, which is connected to the feedback elements through a temperature-
compensating divider circuit.
The feedback elements are connected to the amplifier input and to the ion chamber, and they
conduct away all of the current generated in the ion chamber. The second amplifier stage,
which is fed from the first amplifier, drives the meter through calibrating adjustable resistors.
The gain of the second stage is controlled by the Function Switch so that the full scale output
from the second stage is nearly constant for all ranges.
A common line is used as a reference for all the electronic circuits. This common line is held
approximately 36 volts negative with respect to the instrument chassis.
Figure 3-1: Connector board component layout

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Figure 3-2: Basic operation of the circuit
3.2 Functional Theory
3.2.1 Ion Chamber
The ion chamber is located inside the case below the meter. It consists of the lower two inches
of the 3-in.-diam chamber assembly. The remainder of the volume contains electronic
components. The chamber wall is 0.20 in. (0.51 cm) thick conductive plastic, and the face is
one-mil (25 micron) aluminized Mylar. Another one-mil Mylar layer is glued to the bottom of
the case, making the total thickness two mils, a total of approximately seven mg/cm2. The
active volume of air in the chamber is approximately 220 cm3.
The entire chamber, which is made of conductive plastic, is maintained at chassis potential.
The inside of the Mylar face is also at the same voltage. The center electrode is coated with
conductive graphite and is supported on the center conductor of the guarded feedthrough at
the top of the chamber. The guard ring of the guarded feedthrough is positioned on insulators
between the center conductor and the outer ring, to prevent leakage from the chamber voltage
to the center electrode. The guard ring and center electrode are maintained at the same
(common) potential to prevent leakage from the guard to the electrode.
The chamber is sealed but vented through a plastic tube to a silica gel desiccant package. A
small hole connects the chamber to the electronic section above the chamber. In this way, any
air drawn into the chamber (e.g., as the result of atmospheric pressure changes, temperature
changes, or air transport of the RO-20) must first pass over the desiccant. Dry air in the
chamber is essential to help prevent current leakage.
An idealized air chamber, the size of the one used on the RO-20, produces a little over 2x10–14
amps per mR/h at the standard temperature and pressure of 0 oC and 760 mm of Hg. At 5
mR/h, it should produce 1x10–13 amps and at 50 R/h it should produce 1x10–9 amps. It is seen
that at full scale on the most sensitive range (5 mR/h), about one-tenth of a micro-microamp is
produced in the chamber, which makes protection against leakage currents paramount.

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Note The silica gel desiccant should be changed as soon as it shows clear or pink crystals.
The amount of ion current produced depends upon the intensity of the radiation field and the
density of the air in the chamber. The density depends on the temperature and pressure of the
air. A temperature- sensing compensation circuit corrects for the change in ion current due to
temperature changes, leaving only the pressure effects to be considered external to the
instrument. Table 2-1 and Table 2-2 show correction factors that are useful when the elevation
at calibration is different from the elevation during usage. Normal atmospheric pressure
variations at any one location are small enough to be ignored.
A voltage gradient must exist across the ion chamber to move the ions and produce the ion
current to be measured. All of the chamber structure is maintained at chassis potential, but the
center electrode disk and all the remaining circuitry is biased at –36 volts relative to the
chassis. Three 12-V alkaline batteries provides this voltage. Except for insulation leakage and
the very few microamps taken during battery check, the alkaline battery (3ea 12V) is
essentially open circuited and will last indefinitely.
3.2.2 The Circuit
The first stage of operational amplifier A101 is connected to the ion chamber. (See Figure 3-3
and Figure 3-4.)Also connected to the chamber are the elements in the feedback loop,
consisting of two high- value resistors, a reed relay, and a capacitor. The switching in the loop
is done by two reed relays. One of the relays is used to short out the components so that any
voltage offset may be adjusted out with the Zero Knob. That relay is closed when in the two
battery-check positions (to provide maximum battery load), and in the Zero position of the
Function Switch. It is open in the other Function Switch positions
Figure 3-3: Detector board component layout

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Figure 3-4: Main circuit board layout
When operating on the two R/h ranges, K102 is pulled in to place R102, the 1.5-gigohm
resistor, in the circuit. The higher ion currents develop sufficient voltages for operation with
this smaller resistor. When using the mR/h ranges, both relays are open, leaving R101 (the
150-gigohm resistor) as the feedback element. The feedback signal to the two high- value
resistors comes from the temperature compensation circuit made up of R105, R106, R107 and
RT101. C102 is a time-constant component used to set the response time.
The relays consume about 10 times more power than the total of the rest of the circuits, with
the exception of the lamps. Because no relays are pulled in on the three mR/h ranges, the
battery life is very long if only these ranges are used.
The second stage of A101 amplifies the output of the first stage and then drives the meter
circuit. The gain of the second stage is set according to the Function Switch position. Each
range has a separate calibration control, an adjustable resistor in series with the second stage
output. This circuit then feeds the meter directly through the Function Switch. The Zero Knob
provides a slight voltage offset adjustment to the input of the first stage of A101. The
amplifiers are DC-coupled and this adjustment is reflected at the meter.
Component A203 is a 5-volt regulator whose output is connected to A202, a component
called a rail splitter. It produces a voltage exactly one half of that which is impressed across
it. The voltage it produces is called common and this potential is used as a reference for the
rest of the electronic circuits. The negative end of the three 12V batteries is connected to
common, holding common 36 volts negative with respect to the chassis. The positive terminal
of the meter is connected directly to common so all meter upscale indications must come from
a source more negative than common. When checking the AA cells, amplifier A201-8,9,10 is
used to reflect the opposite polarity of the AA cell battery’s positive end. When checking the
three 12V batteries, amplifier A201-12,13,14 is used to reflect the opposite polarity of the
input from the more positive chassis potential.