Ludlum Measurements 15 User manual

LUDLUM MODEL 15
NEUTRON COUNTER WITH
DETECTOR MODELS 44-7 & 42-14H
June 2020
Serial Number 336330 and Succeeding
Serial Numbers

LUDLUM MODEL 15
NEUTRON COUNTER WITH
DETECTOR MODELS 44-7 & 42-14H
June 2020
Serial Number 336330 and Succeeding
Serial Numbers
This Manual also applies to Serial Numbers of Model 15 Instruments where the accompanying BF-3 detector has been replaced by He-3.
He-3 detectors are labeled with an “H”

Ludlum Measurements, Inc. June 2020
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. June 2020
Table of Contents
Introduction 1
Getting Started 2
Unpacking and Repacking 2-1
Battery Installation 2-2
Connecting a Detector to the Instrument 2-2
Battery Test 2-2
Instrument Test 2-3
Operational Check 2-4
Specifications 3
Model 42-14H Neutron Detector 3-2
Model 44-7 Alpha-Beta-Gamma Detector 3-2
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
Calibration of Discriminator 6-1
Calibration of Ratemeter 6-1
Calibration of High Voltage Readout 6-2
Calibration of High Voltage 6-2
Calibration of High Voltage for Neutrons 6-2
Efficiency Calibration 6-3
Maintenance 6-3
Recalibration 6-3
Batteries 6-4
Troubleshooting 7

Model15 TechnicalManual
Ludlum Measurements, Inc. June 2020
Troubleshooting the Electronics 7-1
Technical Theory of Operation 8
Input 8-1
Amplifier 8-1
Discriminator 8-1
Audio 8-1
Digital Analog Converter 8-2
Scale Ranging 8-2
Meter Drive 8-2
Meter Compensation 8-2
Fast / Slow Time Constant 8-2
Low Voltage Supply 8-3
Low Voltage Reference 8-3
High Voltage Supply 8-3
Recycling 9
Parts List 10
Model 15 Neutron Counter with Detectors 10-1
Main Board, Drawing 464 x 655 10-1
Calibration Board, Drawing 363 ×585 10-3
Wiring Diagram, Drawing 363 × 587 10-4
Drawings and Diagrams 11

Model15 TechnicalManual Section1
Ludlum Measurements, Inc. Page 1-1 June 2020
Introduction
he Model 15 is a Neutron Counter coupled to a Model 42-14H
Neutron Detector and moderator, as well as a Model 44-7 GM
Detector. This combination provides the required electronic
circuitry and detectors for measuring and monitoring alpha, beta,
and gamma radiation, in addition to fast and slow neutrons (removable
moderator for thermal neutrons).
The instrument provides four linear ranges used in combination with a CPM
(counts per minute) meter dial. The instrument features a regulated 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.
Other controls include: two-position toggle switch for selection of alpha-
beta-gamma (B-G) or neutron detection (N), high-voltage check (HV) and
meter reset (RES) buttons, and front-panel calibration controls for
independent high voltage and discrimination adjustments.
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), either very fresh alkaline or rechargeable NiCd
batteries should be used.
Section
1
T

Model15 TechnicalManual Section2
Ludlum Measurements, Inc. Page 2-1 June 2020
Getting Started
Unpacking and Repacking
Remove the calibration certificate and place it in a secure location. Remove
the instrument and accessories (batteries, cable, detectors, 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 15 serial number is located on the front panel below the battery
compartment. Most Ludlum Measurements 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.
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.
Model 44-7
When shipping a Model 44-7 by air, it is necessary to ship the
tube in a sealed container to avoid sudden atmospheric
changes, which could rupture the tube.
Section
2

Model15 TechnicalManual Section2
Ludlum Measurements, Inc. Page 2-2 June 2020
Battery Installation
Ensure the Model 15 range selector switch is in the OFF position. Open the
battery lid by pushing down and turning the quarter-turn thumbscrew
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, then 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 a Detector to the Instrument
Caution!
The detector operating voltage (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 15 range selector switch to the
OFF position before connecting or disconnecting the cable or
detector.
Connect one end of a detector cable to the desired 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.

Model15 TechnicalManual Section2
Ludlum Measurements, Inc. Page 2-3 June 2020
Instrument Test
Connect the GM detector if you have not already done so. Set the B-G/N
switch to B-G. After checking the batteries, turn the instrument range switch
to the ×1000 position. Place the AUD ON-OFF switch in the ON position.
Expose the detector to a radiation check source. 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.
Press the HV check button and confirm that the meter indicates
approximately 900 volts (same as indicated on calibration test sheet). Rotate
the range switch through the lower scales until a meter reading is indicated.
Record the reading.
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 five 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 zero.
Turn the instrument off. Disconnect the GM detector and connect the
neutron detector. Set the B-G/Nswitch to N. Press the HV check button and
compare the high-voltage reading to the calibration test sheet reading
(approximately 1275-1350 volts).
If a neutron source is available, expose the detector to the source (at a
repeatable distance) and record the reading. Daily source checks may
then be performed by orienting the detector and source in identical
fashion, ensuring an instrument reading of ±20% of the initial reading.
Failure to meet the ±20% criteria may indicate a detector malfunction.
Once this procedure has been completed, the instrument is ready for use.