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.
Sectio
n
1
T

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
Sectio
n
2

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
Sectio
n
3