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This Manual also applies to Serial Numbers of Model 15 Instruments where the accompanying BF-3 detector has been replaced by He-3.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

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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.

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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.

Model15 TechnicalManual Section2
Ludlum Measurements, Inc. Page 2-4 June 2020
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.

Model15 TechnicalManual Section3
Ludlum Measurements, Inc. Page 3-1 June 2020
Specifications
Instrument High Voltage: adjustable independently from 200 to 2500
V; can be read on the meter; electronically regulated to 1%; support of
scintillation loads to 1500 V and proportional loads to 2500 V
Input Pulse Threshold: adjustable from -2 to -60 mV
Input Impedance: 0.1 megohm
Meter: 6.4 cm (2.5 in.) arc, pivot-and-jewel suspension
Meter Face: 0-500 cpm, 0-2.5 kV, BAT TEST (others available)
Instrument Range: typically 0-500,000 cpm, depending on the meter
face utilized
Linearity: reading within 10% of true value with detector connected
Audio: built-in unimorph speaker with ON-OFF switch (greater than
60 dB at 0.61 m {2 feet})
Connector: series ˝C˝(other 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)
Battery Dependence: instrument calibration changes less than 3%
within battery check limits on the meter
Size: 31.8 x 8.9 x 21.6 cm (12.5 x 3.5 x 8.5 in.) (H x W x L)
Weight: 3.7 kg (8.1 lb), including batteries
Construction: cast-and-drawn aluminum with beige powder coating
Section
3

Model15 TechnicalManual Section3
Ludlum Measurements, Inc. Page 3-2 June 2020
Cable: 99.1 cm (39 in.) with ˝C˝connector (other available)
Neutron Detector: 2 Atm 3He tube, LND 25185 or equivalent;
proportional detector with 7.6 cm (3 in.) diameter, cadmium-lined
moderator for fast neutrons (remove detector from moderator for
thermal neutrons)
Neutron Detector Operating Voltage: approximately 1275-1350 Vdc
Energy Response: count response nonlinear throughout energy
spectrum (0.025-12 MeV)
Neutron Sensitivity: typically 55 cpm/mrem/hr (241AmBe fast
neutrons)
Detection Range: thermal to approximately 12 MeV
Gamma Rejection: typically 10 cpm or less through 10 R/hr (100
mSv/h) (137Cs)
Alpha-Beta-Gamma Detector: thin end-window GM detector
Window: 2.0 ±0.3 mg/cm2mica
Window Area: active is 6.4 cm2 (1 in2); open is 5.2 cm2 (0.8 in2)
Efficiency (4pi geometry): typically 2% for 14C, 10% for 90Sr90Y, 7% for
239Pu, and 7% for 99Tc
Alpha-Beta-Gamma Sensitivity: typically 2100 cpm per mR/hr (137Cs
gamma)
Background: typically 40 cpm
Model 42-14H
Neutron Detector
Model 44-7 Alpha-Beta-
Gamma Detector

Model15 TechnicalManual Section4
Ludlum Measurements, Inc. Page 4-1 June 2020
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 0-500 cpm, 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
TEST 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 500,000 cpm. Multiply the scale reading by
the multiplier to determine the actual scale reading.
Battery Compartment: sealed compartment to house two ˝D˝cell
batteries.
AUD ON-OFF Toggle Switch: in the ON position, 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.
RES Pushbutton: When depressed, this button provides a rapid means
of driving the meter needle to zero.
F/S 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.
Section
4

Model15 TechnicalManual Section4
Ludlum Measurements, Inc. Page 4-2 June 2020
B-G/N: This switch allows for selection of the desired detector and is
associated with the HV outputs. In the B-Gposition, the HV is adjusted
for approximately 900 volts for use with GM detectors. In the N
position, the HV is adjusted for approximately 1275-1350 volts when
utilizing the Model 42-14H Neutron detector.
HV Pushbutton Switch: When depressed, displays the detector high
voltage on the meter face, corresponding to the selected detector (B-
G/N). 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.
The following controls are located under the front-panel
calibration cover:
Discriminator Adjustment (DIS): allows the input pulse threshold to be
adjusted from 2 to 60 mV.
HVB Adjustment: provides a means of varying the alpha-beta-gamma
high voltage from 200 to 2500 V; factory set at 900 V
Range Calibration Adjustments: These adjustment controls allow
individual calibration for each range multiplier.

Model15 TechnicalManual Section5
Ludlum Measurements, Inc. Page 5-1 June 2020
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 body 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.
Section
5

Model15 TechnicalManual Section5
Ludlum Measurements, Inc. Page 5-2 June 2020
The Model 15 Neutron Counter 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 15 instrument and associated detectors 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.

Model15 TechnicalManual Section6
Ludlum Measurements, Inc. Page 6-1 June 2020
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:
Measure high voltage with a Ludlum Model 500 Pulser or a
high-impedance voltmeter with a high-meg probe. If one of
these instruments is not available, use a voltmeter with a
minimum input resistance of 1000 megohms.
Calibration of Discriminator
Set the instrument range switch to X10. Remove the calibration cover
plate. Connect the Model 15 to the Model 500 Pulser. Set the pulser’s
pulse height to 10 ±0.5 mV for use without a GM detector. If a GM
detector is used, set the pulse height to 30 ±0.5 mV. Set the pulser’s
pulse rate to 400 × 10 counts per minute.
Adjust the DIS control until the meter reading increases from 0 to a
stable reading near 400 counts per minute. Check the adjustment by
lowering the Model 500 pulser’s pulse height to 9 mV for 10 mV setting
or 29 mV for 30 mV setting. Meter reading should drop to 0. Increase
the Model 500 pulse height to 12 mV for a 10 mV DIS setting or 32 mV
for a 30 mV DIS setting. Meter reading should increase to a stable value
near 400 counts per minute.
Calibration of Ratemeter
Set the Range switch to X1000. Set the pulser’s pulse rate to 400 × 1K
counts per minute. Set the pulser’s pulse height to 20 mV for a DIS
setting of 10 mV or 60 mV for a DIS setting of 30 mV.
Section
6

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Ludlum Measurements, Inc. Page 6-2 June 2020
Now adjust the X1000 calibration potentiometer until the Model 15
reads 400 × 1000 counts/minute. Reduce the pulser’s pulse rate to 100
× 1K counts per minute. Confirm that meter reading is 100 ±10.
Set the pulser’s pulse rate to 400 × 100 counts per minute. Set the
Model 15 range switch to X100. Adjust the X100 calibration
potentiometer until the Model 15 reads 400 ×100 counts per minute.
Reduce the counts on Pulser to 100 × 100 counts per minute and
confirm that the Model 15 reads 100 ±10. Repeat the procedure for the
X10 and X1 ranges.
Calibration of High Voltage Readout
Open the instrument and locate the HV calibration control on the main
circuit board. Connect the Model 15 to the Model 500 Pulser. Set the B-
G/N switch to N. Press the HV Test switch and record the HV reading
on both the Model 500 and the Model 15.
Adjust the HVN potentiometer for 1500 V on pulser readout. Press the
HV button and adjust the internal Model 15 HV control until the meter
reads 1500 V. Reduce the HVN control for 1000 V on pulser HV readout.
HV test should indicate 1000 ±10%. Increase the HVN control to 2000 V
on Pulser HV readout. HV test should indicate 2000 ±10% V.
Calibration of High Voltage
(For Beta Gamma Set B-G/Nto B-G.) Press the HV test switch and adjust
the HVB calibration potentiometer for a reading of 900 V.
Calibration of High Voltage For Neutrons
Switch the B-G/Nswitch to N. Connect the neutron probe. Press the HV
test switch. Record readings in a known neutron field as the high voltage
is varied from approximately 1200-1500 V in 50 V steps.
Expose Neutron detector to a 1 R/hr gamma field. Reduce the high-
voltage setting until count rate is less than 10 counts per minute.

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Ludlum Measurements, Inc. Page 6-3 June 2020
Plot the data from the previous two steps. Pick an operating point just
above the knee of the count plateau, but at a voltage less than the Gamma
cutoff found in the previous step. Record this voltage and adjust the HVN
calibration potentiometer for operation at this voltage.
Efficiency Calibration
With the instrument fully calibrated, set the B-G/Nswitch to N. Connect
the neutron detector within the moderator. Expose the detector to a
known neutron field and determine counts/minute per mrem/hr for that
energy source. Note that the Model 15 is energy dependent.
Repeat the above procedure with the B-G/Nswitch in the B-Gposition and
the gamma detector connected and exposed to a known gamma field.
Determine counts/minute per mR/hr.
Maintenance
Instrument maintenance consists of keeping the instrument clean and
periodically checking the batteries and the calibration. The Model 15
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.
Recalibration
Recalibration should be accomplished after maintenance or adjustments have
been performed on the instrument. Recalibration is not normally required
following instrument cleaning, battery replacement, or detector cable
replacement of equal length.
Note:
Ludlum Measurements, Inc. recommends recalibration at
intervals no greater than one year. Check the appropriate
regulations to determine required recalibration intervals.