LUDLUM 12-4 User manual

LUDLUM MODEL 12-4
SURVEY METER WITH
HE-3 NEUTRON DETECTOR
May 2016
Serial Number 227878 and Succeeding
Serial Numbers

LUDLUM MODEL 12-4
SURVEY METER WITH
HE-3 NEUTRON DETECTOR
May 2016
Serial Number 227878 and Succeeding
Serial Numbers
This Manual also applies to Serial Numbers of Model 12-4 Instruments where the BF-3 detector has been modified to He-3.
He-3 detectors, and those modified to be, are labeled with an “H.”

STATEMENT OF WARRANTY
Ludlum Measurements, Inc. warrants the products covered in this manual to be free of
defects due to workmanship, material, and design for a period of twelve months from the
date of delivery. The calibration of a product is warranted to be within its specified
accuracy limits at the time of shipment. In the event of instrument failure, notify Ludlum
Measurements to determine if repair, recalibration, or replacement is required.
This warranty excludes the replacement of photomultiplier tubes, G-M and proportional
tubes, and scintillation crystals which are broken due to excessive physical abuse or used
for purposes other than intended.
There are no warranties, express or implied, including without limitation any implied
warranty of merchantability or fitness, which extend beyond the description of the face
there of. If the product does not perform as warranted herein, purchaser’s sole remedy
shall be repair or replacement, at the option of Ludlum Measurements. In no event will
Ludlum Measurements be liable for damages, lost revenue, lost wages, or any other
incidental or consequential damages, arising from the purchase, use, or inability to use
product.
RETURN OF GOODS TO MANUFACTURER
If equipment needs to be returned to Ludlum Measurements, Inc. for repair or calibration, please send to
the address below. All shipments should include documentation containing return shipping address,
customer name, telephone number, description of service requested, and all other necessary information.
Your cooperation will expedite the return of your equipment.
LUDLUM MEASUREMENTS, INC.
ATTN: REPAIR DEPARTMENT
501 OAK STREET
SWEETWATER, TX 79556
800-622-0828 325-235-5494
FAX [phone]

Ludlum Measurements, Inc. May 2016
Table of Contents
Introduction 1
Getting Started 2
Unpacking and Repacking 2-1
Battery Installation 2-1
Connecting the Detector to the Instrument 2-2
Battery Test 2-2
Instrument Test 2-3
Operational Check 2-3
Specifications 3
Identification of Controls and Functions 4
Safety Considerations 5
Environmental Conditions for Normal Use 5-1
Warning Markings and Symbols 5-1
Cleaning and Maintenance Precautions 5-2
Calibration and Maintenance 6
Calibration 6-1
Establishing an Operating Point 6-1
Equipment Required for Calibration 6-1
Initial Calibration Procedures 6-2
Detector Plateau 6-3
Gamma Rejection 6-4
Instrument Calibration 6-4
Electronic Calibration 6-5
Maintenance 6-5
Recalibration 6-6
Batteries 6-6
Troubleshooting 7
Troubleshooting Electronics that utilize a Proportional Detector 7-1

Model12-4 TechnicalManual
Ludlum Measurements, Inc. May 2016
Technical Theory of Operation 8
Main Board (Drawing 464 × 275, 3 sheets) 8-1
Input 8-1
Amplifier 8-1
Discriminator 8-1
Audio 8-1
Scale Ranging 8-2
Digital Analog Converter 8-2
Ratemeter Drive Circuit 8-2
HV Meter Drive 8-2
Battery Test 8-2
HV Power Supply Board (Drawing 464 × 243) 8-3
Switching Convertor 8-3
Feedback 8-3
Voltage Control 8-3
Filtering 8-3
Recycling 9
Parts List 10
Model 12-4 Survey Meter 10-1
Main Board, Drawing 464 × 275 10-1
HV Power Supply Board, Drawing 464 × 243 10-4
Wiring Diagram, Drawing 464 × 309 10-5
Model 42-31H Neutron Detector 10-5
Drawings and Diagrams 11

Model12-4 TechnicalManual Section1
Ludlum Measurements, Inc. Page 1-1 May 2016
Introduction
he Model 12-4 is a Survey Meter coupled to a Model 42-31H
Neutron Detector; providing the required electronic circuitry and
detector for measuring and monitoring of neutron radiation. The
instrument provides four linear ranges used in combination with an
exposure rate or CPM (counts per minute) meter dial. The instrument
features a regulated HV (high-voltage) power supply, unimorph speaker
with audio ON - OFF capability, fast-slow meter response, meter reset button
and a six-position switch for selecting battery check or range multiples of
×1, ×10, ×100, and ×1000. Each range multiplier has its own calibration
potentiometer. The unit body and meter housing are made of cast aluminum
and the can is 0.23 cm (0.090 in.) thick aluminum.
The unit is powered by two ˝ D ˝cell batteries for operation from -20 to 50 ˚C
(-4 to 122 ˚F). For instrument operation below 0˚C (32 ˚F), use very fresh
alkaline or rechargeable NiCd batteries.
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Model12-4 Technical Manual Section 2
Ludlum Measurements, Inc. Page 2-1 May 2016
Getting Started
Unpacking and Repacking
Remove the calibration certificate and place it in a secure location. Remove
the instrument and accessories (batteries, cable, etc.) and ensure that all of
the items listed on the packing list are in the carton. Check individual item
serial numbers and ensure calibration certificates match. The Model 12-4
serial number is located on the front panel below the battery compartment.
Most Ludlum Measurements, Inc. detectors have a label on the base or body
of the detector for model and serial number identification.
Important!
If multiple shipments are received, ensure that the detectors
and instruments are not interchanged. Each instrument is
calibrated to specific detectors, and therefore not
interchangeable.
To return an instrument for repair or calibration, provide sufficient packing
material to prevent damage during shipment. Also provide appropriate
warning labels to ensure careful handling.
Every returned instrument must be accompanied by an Instrument Return
Form, which can be downloaded from the Ludlum website at
www.ludlums.com. Find the form by clicking the “Support” tab and
selecting “Repair and Calibration” from the drop-down menu. Then choose
the appropriate Repair and Calibration division where you will find a link to
the form.
Battery Installation
Ensure the Model 12-4 range selector switch is in the OFF position. Open
the battery lid by pushing down and turning the quarter-turn thumbscrew
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Model12-4 Technical Manual Section 2
Ludlum Measurements, Inc. Page 2-2 May 2016
counterclockwise a quarter of a turn. Install two ˝ D ˝size batteries in the
compartment.
Note the (+) and (-) marks inside the battery door. Match the battery
polarity to these marks. Close the battery box lid, push down, and turn the
quarter-turn thumb screw clockwise a quarter of a turn.
Note:
The center post of a flashlight battery is positive. The batteries
are placed in the battery compartment in opposite directions.
Connecting the Detector to the Instrument
Caution!
The detector operating voltage (also referred to as HV) is
supplied to the detector via the detector input connector. A
mild electric shock may occur if you make contact with the
center pin of the input connector. Switch the Model 12-4
range selector switch to the OFF position before connecting or
disconnecting the cable or detector.
Connect one end of a detector cable to the detector by firmly pushing the
connectors together while twisting clockwise a quarter of a turn. Repeat the
process in the same manner with the other end of the cable and the
instrument.
Battery Test
The batteries should be checked each time the instrument is turned on.
Move the range switch to the BAT position. Ensure that the meter needle
deflects to the battery check portion on the meter scale. If the meter does
not respond, check to see if the batteries have been correctly installed.
Replace the batteries if necessary.

Model12-4 Technical Manual Section 2
Ludlum Measurements, Inc. Page 2-3 May 2016
Instrument Test
After checking the batteries, turn the instrument range switch to the ×1000
position. Place the AUD ON-OFF switch in the ON position. The instrument
speaker should emit ˝clicks˝relative to the rate of counts detected. The AUD
ON/OFF switch will silence the audible clicks if in the OFF position.
It is recommended that the AUD ON/OFF switch be kept in the OFF position
when not needed in order to preserve battery life.
Rotate the range switch through the lower scales until a meter reading is
indicated. While observing the meter fluctuations, select between the fast
and slow response time (F/S) positions to observe variations in the display.
The ˝S˝position should respond approximately 5 times slower than the ˝F˝
position.
Note:
The slow response position is normally used when the
instrument is displaying low numbers, which require a more
stable meter movement. The fast response position is used at
high rate levels.
Check the meter reset function by depressing the RES pushbutton switch
and ensuring the meter needle drops to “0.”
Once this procedure has been completed, the instrument is ready for use.
Operational Check
To assure proper operation of the instrument – between calibrations and
periods of nonuse – an instrument operational check, including battery test
and instrument test (as described above), should be performed prior to use.

Model12-4 Technical Manual Section 3
Ludlum Measurements, Inc. Page 3-1 May 2016
Specifications
Operating Voltage: approximately 1200 Vdc
Detector: 2 Atm 3He tube – LND 25185 or equivalent – proportional
detector, surrounded by a 22.9 cm (9 in.) diameter polyethylene sphere
Detector Input Sensitivity: -2 mV
Sensitivity: typically 10,000 cpm per µSv/h (100 cpm/mrem/hr)
(241AmBe fast neutrons)
Detection Range: thermal to approximately 12 MeV, provides an
approximate inverse RPG curve (see below)
Meter: 6.4 cm (2.5 in.) arc, pivot-and-jewel suspension.
Meter Face: 0-10 mrem/hr, 0-2.5 kV, BAT TEST (others available)
Instrument Range: typically 0-100 mSv/h (0-10,000 mrem/hr),
depending on the meter face utilized
Gamma Rejection: typically 10 cpm or less through 0.1 Sv/h (10 R/hr)
(137Cs)
Input Impedance: 0.1 megohm
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Model12-4 Technical Manual Section 3
Ludlum Measurements, Inc. Page 3-2 May 2016
Linearity: reading within 10% of true value with detector connected
Discriminator: adjustable from 1 to 50 mV
Audio: built-in unimorph speaker with ON-OFF switch (greater than
60 dB at 61 cm {2 ft})
Response: toggle switch for FAST (4 seconds) or SLOW (22 seconds)
from 10% to 90% of final reading
Connector: Series ˝C˝(other available)
Cable: 99.1 cm (39 in.) with ˝C˝connector (others available)
Power: two ˝D˝cell batteries housed in a sealed externally accessible
compartment
Battery Life: typically 600 hours with alkaline batteries (battery
condition may be checked on the meter)
End-of-Battery Life Warning: At 2.1 Vdc (voltage direct current) the
meter needle will drop to the edge of the BAT TEST or BAT OK area when
the meter selector switch is moved to the BAT position. At 2.0 Vdc a
steady audible tone will be emitted to warn the user of the low battery
condition.
Battery Dependence: instrument calibration change less than 3%
within battery check limits on the meter
Size: 43.2 x 22.9 x 26.7 cm (17 x 9 x 10.5 in.) (H x W x L)
Weight: 7.9 kg (17.5 lb)
Construction: cast and drawn aluminum with beige powder-coat finish

Model12-4 Technical Manual Section 4
Ludlum Measurements, Inc. Page 4-1 May 2016
Identification of Controls and
Functions
Meter: 6.4 cm (2.5 in.) arc, 1 mA analog type with pivot-and-jewel
suspension. Typical meter dial is 0-10 mrem/hr, 0-2.5 kV and BAT TEST.
Connector: Used to connect the detector to the instrument. Typically
series ˝C,˝but can be ˝BNC˝, ˝MHV˝, ˝UHF,˝or others.
Range Selector Switch: a six-position switch marked OFF, BAT, ×1000,
×100, ×10, and ×1. Turning the range selector switch from OFF to BAT
provides the operator with a battery check of the instrument. A BAT
check scale on the meter provides a visual means of checking the
battery-charge status. Moving the range selector switch to one of the
range multiplier positions (×1000, ×100, ×10, ×1) provides the operator
with an overall range of 0 to 10,000 mrem/hr. Multiply the scale reading
by the multiplier to determine the actual scale reading.
Discriminator Adjustment: allows the input sensitivity to be adjusted
from 1 to 100 mV; factory set at -2 mV.
HV Adjustment: provides a means of varying the high voltage from 200
to 2500 V; factory set at 1100 V.
Range Calibration Adjustment: recessed potentiometers located under
the calibration cover on the right side of the front panel. These
adjustment controls allow individual calibration for each range
multiplier.
Battery Compartment: sealed compartment to house two ˝D˝cell
batteries.
AUD ON-OFF Toggle Switch: In the ON position, it operates the
unimorph speaker located on the left side of the instrument. The
frequency of the clicks is relative to the rate of the incoming pulses. The
higher the rate, the higher the audio frequency. This switch should be
turned OFF when not required in order to reduce battery drain.
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Model12-4 Technical Manual Section 4
Ludlum Measurements, Inc. Page 4-2 May 2016
RES Pushbutton: When depressed, this button provides a rapid means
of driving the meter needle to zero.
Toggle Switch: provides meter response. Selecting the fast, ˝F˝position
of the toggle switch provides 90% of full-scale meter deflection in four
seconds. In the slow, ˝S˝position, 90% of full-scale meter deflection
takes 22 seconds. In the ˝F˝position there is fast response and large
meter deviation. The ˝S˝position should be used for slow response and
damped meter deviation.
HV Pushbutton Switch: When depressed, displays the detector high
voltage on the meter. The output resistance of the high voltage-supply is
1.5 megohms with a typical scintillation voltage divider of 100 megohms.
The actual detector voltage will be 98.5% of the indicated voltage.

Model12-4 Technical Manual Section 5
Ludlum Measurements, Inc. Page 5-1 May 2016
Safety Considerations
Environmental Conditions for Normal Use
Indoor or outdoor use
No maximum altitude
Temperature range of -20 to 50 °C (-4 to 122 °F); may be certified for
operation from -40 to 65 °C (-40 to 150 °F)
Maximum relative humidity of less than 95% (non-condensing)
Pollution Degree 3 (as defined by IEC 664) (Occurs when conductive
pollution or dry nonconductive pollution becomes conductive due to
condensation. This is typical of industrial or construction sites.)
Warning Markings and Symbols
Caution!
The operator or responsible party is cautioned that the
protection provided by the equipment may be impaired if the
equipment is used in a manner not specified by Ludlum
Measurements, Inc.
Caution!
Verify instrument voltage input rating before connecting to a
power converter. If the wrong power converter is used, the
instrument and/or power converter could be damaged.
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Model12-4 Technical Manual Section 5
Ludlum Measurements, Inc. Page 5-2 May 2016
The Model 12-4 Survey Meter is marked with the
following symbols:
CAUTION, RISK OF ELECTRIC SHOCK (per ISO 3864, No. B.3.6):
designates a terminal (connector) that allows connection to a voltage
exceeding 1 kV. Contact with the subject connector while the instrument is
on or shortly after turning off may result in electric shock. This symbol
appears on the front panel.
CAUTION (per ISO 3864, No. B.3.1): designates hazardous live voltage
and risk of electric shock. During normal use, internal components are
hazardous live. This instrument must be isolated or disconnected from the
hazardous live voltage before accessing the internal components. This
symbol appears on the front panel. Note the following precautions:
Warning!
The operator is strongly cautioned to take the following
precautions to avoid contact with internal hazardous live parts
that are accessible using a tool:
1. Turn the instrument power OFF and remove the batteries.
2. Allow the instrument to sit for one minute before accessing
internal components.
The “crossed-out wheelie bin” symbol notifies the consumer that the
product is not to be mixed with unsorted municipal waste when discarding;
each material must be separated. The symbol is placed on the battery
compartment lid. See Section 9, “Recycling,” for further information.
The “CE” mark is used to identify this instrument as being acceptable for
use within the European Union.
Cleaning and Maintenance Precautions
The Model 12-4 instrument and detector may be cleaned externally with a
damp cloth, using only water as the wetting agent. Do not immerse the
instrument in any liquid. Observe the following precautions when cleaning
or performing maintenance on the instrument:
1. Turn the instrument OFF and remove the batteries.
2. Allow the instrument to sit for one minute before cleaning the
exterior or accessing any internal components for maintenance.

Model12-4 Technical Manual Section 6
Ludlum Measurements, Inc. Page 6-1 May 2016
Calibration and Maintenance
Calibration
Calibration controls are located on the front of the instrument under the
calibration cover. The controls may be adjusted with a 1/8-inch blade
screwdriver.
Note:
Local procedures may supersede the following.
Establishing an Operating Point
The operating point for the instrument and detector is established by setting
the detector voltage and instrument sensitivity (HV and DIS). The proper
selection of this point is the key to instrument performance. Efficiency,
background sensitivity, and noise are fixed by the physical makeup of the
given detector and rarely vary from unit to unit. However, the selection of
the operating point makes a marked difference in the apparent contribution
of these three sources of count.
In setting the operating point, the final result of the adjustment is to establish
the system gain so that the desirable signal pulses (including background) are
above the discrimination level and the unwanted pulses from noise are
below the discrimination level and are therefore, not counted.
Adjusting either the instrument gain or the HV controls the total system
gain. Voltage affects control in the detector; DIS (Discriminator) controls the
amplifier gain.
Equipment Required for Calibration
All instruments used in calibrating the Model 12-4 must be calibrated by
standards traceable to the National Institute of Standards and Technology
and must have a current calibration label attached.
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Model12-4 Technical Manual Section 6
Ludlum Measurements, Inc. Page 6-2 May 2016
A Ludlum Model 500 Pulser or equivalent is required. If the
pulser does not have an HV readout, use a high-impedance
voltmeter with at least 1000 Megohm meter input resistance to
adjust the detector voltage.
A neutron source capable of yielding dose-equivalent rates
ranging from 2 mrem/hr to 1 rem/hr in needed.
A 137Cs source capable of yielding an exposure rate of 10 R/hr is
needed.
Initial Calibration Procedures
If any calibration procedure cannot be completed satisfactorily, the
instrument shall be tagged and removed for proper disposition.
Note:
When placing the Model 12-4 into its can, orient the
instrument so that the unimorph speaker is on the left side
when reading the meter dial. Noise can be fed into the preamp
of the instrument if it is placed backward into the can.
Ensure that the meter movement has proper mechanical zero. The
adjustment is on the front of the meter bezel. It must be adjusted to
"zero" with the Range Selector Switch in the OFF position.
Perform geotropism check.
Connect the Model 12-4 to the pulser or a certified high-impedance
voltmeter.
Rotate the range selector switch to the BAT position. The meter pointer
should remain in the BAT TEST area of the meter dial.
Rotate the Model 12-4 Range Selector Switch to ×1000.
Adjust the CAL HV potentiometer on the Model 12-4 for a reading of
1.5 kV on the pulser HV meter.

Model12-4 Technical Manual Section 6
Ludlum Measurements, Inc. Page 6-3 May 2016
Warning!
Avoid electrical shock! Do not allow any conductive material
to come in contact with internal parts.
Depress the Model 12-4 HV pushbutton. The Model 12-4 meter pointer
should be over the 1.5 mark on the kV dial scale. If it is not, remove the
Model 12-4 from the can and adjust the HV potentiometer (R49) on the
main board until the pointer is over the 1.5 kV mark. Place the Model
12-4 back in the can, with the unimorph speaker on the left side.
Depress the Model 12-4 HV button and adjust the Model 12-4 HV CAL
potentiometer (front panel) until the Model 12-4 meter indicates 0.5 kV.
The pulser voltmeter should read 0.5 kV ± 10%.
Depress the Model 12-4 HV button and adjust the Model 12-4 HV CAL
potentiometer (front panel) until the Model 12-4 meter indicates 2.0 kV.
The pulser voltmeter should read 2.0 ±10% kV.
Reset the Model 12-4 HV to 1100 volts.
Set the pulser for -2 mV.
Adjust DIS for a stable meter reading, and then back off the control until
the meter reading declines 10%.
Detector Plateau
Secure a Model 2200 (or equivalent instrument) with input
discrimination adjusted to -2 mV.
Connect the Model 12-4 detector to the Model 2200. Background count
should be less than 1 cpm (counts per minute).
Place the Model 12-4 detector assembly in a neutron field. Expose the
detector to a 20 mrem/hr neutron field. Run a plateau of cpm versus
detector high voltage, within a suggested range of 1000-1250 volts.
Use 25-volt steps.

Model12-4 Technical Manual Section 6
Ludlum Measurements, Inc. Page 6-4 May 2016
From your data, pick a high-voltage operating point between 2000 and
2800 cpm at 20 mrem/hr.
The operating point should show less than 10% deviation of count for
±25 volt change.
Record this count at your selected operating point. Record the sensitivity
in counts per minute/mrem/hr. Record the high-voltage operating
point.
Gamma Rejection
Remove the Model 42-31H detector from the moderator and place in a
10 R/hr 137Cs gamma radiation field.
Take a one-minute count. If more than 10 counts are observed for the
one-minute period, decrease the operating voltage until the count rate
drops below 10 cpm; however, ensure that the HV remains in the
plateau region determined above.
Ensure that sensitivity for the selected operating voltage is
approximately 120 cpm/mrem/hr.
Return the Model 42-31H to the moderator.
Instrument Calibration
Connect the Model 12-4 counting instrument to a Model 500 Pulser.
Now adjust the Pulser to 4-6 mV.
Adjust the Model 12-4 HV control for a HV setting equal to the operating
point HV from the previous paragraph.
Multiply the sensitivity in counts/minute/mrem/hr times 80 for the
specific detector.
Adjust the Model 500 pulse rate to the calculated value. (Count rate
should be 9600 cpm ±10%.) Set the Model 12-4 range selector switch to
×10 and adjust the ×10 calibration potentiometer for a meter reading of
˝8˝.
Connect the Model 12-4 counter to the Model 12-4 detector assembly.

Model12-4 Technical Manual Section 6
Ludlum Measurements, Inc. Page 6-5 May 2016
Expose the detector to an 80 mrem/hr neutron field.
Confirm that the meter reads 8 mrem/hr on the ×10 scale.
If the reading is not 8 mrem/hr ±5%, make a slight adjustment to the HV
supply to yield an 8 mrem/hr meter reading.
Expose the detector to a 20 mrem/hr field and confirm a meter reading of
20 mrem/hr ± 10%.
Proceed to calibrate all scales with a meter reading of 2 and 8 mrem/hr.
Electronic Calibration
If your range is inadequate to equal all points between 2 mrem/hr and 800
mrem/hr, you may use a pulser to electronically calibrate the instrument.
Multiply the cpm/mrem/hr as determined on the plateau
check by the range selection and meter reading. (i.e. typically,
120 cpm/mrem/hr × 1000 × 8 = 960,000 cpm for 800
mrem/hr)
Set this calculated number in the pulser and calibrate the
associated scale.
Note on the calibration certificate that this range was
electronically calibrated.
Maintenance
Instrument maintenance consists of keeping the instrument clean and
periodically checking the batteries and the calibration. The Model 12-4
instrument may be cleaned with a damp cloth (using only water as the wetting
agent). Do not immerse instrument in any liquid. Observe the following
precautions when cleaning:
1. Turn the instrument OFF and remove the batteries.
2. Allow the instrument to sit for one minute before accessing
internal components.