Ludlum Measurements 2241-4 User manual

LUDLUM MODEL 2241-4
SURVEY METER WITH
NEUTRON DETECTOR
October 2022
Serial Number 170658 and Succeeding
Serial Numbers

LUDLUM MODEL 2241-4
SURVEY METER WITH
NEUTRON DETECTOR
October 2022
Serial Number 170658 and Succeeding
Serial Numbers
Model 2241-4 with
Model 42-31H Detector

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. October 2022
Table of Contents
Introduction 1
Getting Started 2
Unpacking and Repacking 2-1
Battery Installation 2-2
Detector Connection 2-2
Operational Check 2-2
Specifications 3
Instrument 3-1
Removable Switchboard Adjustable Parameters 3-2
Model 42-31H Neutron Detector 3-3
Identification of Controls and Functions 4
Display 4-1
Display Status Definitions 4-1
Front Panel Controls 4-2
Front Panel Calibration Controls 4-3
Main Board Controls 4-4
Switch Board Controls 4-5
Safety Considerations 5
Environmental Conditions for Normal Use 5-1
Detector Connector 5-1
Warning Markings and Symbols 5-1
Maintenance 6
Recalibration 6-1
Batteries 6-2
Technical Theory of Operation 7
Main Board 7-1
Switch Board 7-3
Display Board 7-3

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Instrument Setup & Calibration 8
Entering or Changing Switch Board Parameters 8-1
Loading Default Parameters 8-1
The Function Switch 8-2
Function Switch Position Descriptions and Variables 8-3
Calibration 8-8
General Detector Setup Information 8-8
Counts per Minute (C/m) Calibration 8-10
R/hr Calibration 8-11
Detector CC and DTC 8-12
Software License Agreement 8-19
Recycling 9
Parts List 10
Model 2241-4 Survey Meter 10-1
Main Circuit Board, Drawing 408 × 223 10-1
Calibration Board, Drawing 408 × 12 10-3
Display Board, Drawing 408 × 259 10-4
Switch Board, Drawing 408 × 45 10-5
Chassis Wiring Diagram, Drawing 408 × 103 10-5
Drawings and Diagrams 11
RS-232 Output Formats - Appendix A A
Binary Output Format A-1
ASCII Output Format A-4

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Introduction
he Model 2241-4 is a portable microprocessor-based digital Scaler/
Ratemeter coupled to a Model 42-31H Neutron Detector for use in
measuring and monitoring neutron radiation. Data is presented on a
four-digit (six digits in the scaler mode) Liquid Crystal Display
(LCD) with moving decimal point. A three-position switch labeled
˝OFF/RATEMETER/SCALER˝ selects the desired operating mode for the
instrument.
Programmable display units (RATEMETER mode only) are represented in
R/hr (normally represents roentgens/hr, but in this application represents
rem/hr), Sv/h, cpm, or cps with multipliers of micro (µ) or milli (m) for
R/hr and Sv/h and kilo (k) for cpm or cps. The display units are auto-
ranging, enabling the readout to display a broad range of radiation levels.
The display also offers lower limit capability. For example, the display can be
set to show only values that are greater than or equal to1 µR/hr.
This instrument incorporates independent adjustable alarms for RATEMETER
and SCALER operating modes. The RATEMETER mode has two alarm
indicators. The first-level alarm is indicated by display of the word ˝ALERT˝
on the LCD. The second-level alarm is indicated by display of the word
˝ALARM˝and by the emitting of a continuous audible tone. The SCALER
alarm condition will also display the word ˝ALARM˝and produce the same
audible tone. Both audible alarms may be silenced (acknowledged) by
depressing the RESET switch. All alarms are concurrent.
Other features include Dead Time Correction (DTC) to compensate for
detector dead time; audible click-per-event with programmable 1, 10, 100,
and 1000 divide-by; LCD backlight with programmable ON time;
programmable fixed or variable response time; and count overflow visual
alarm, indicating that the counting circuitry is nearing the maximum
counting capability.
Section
1
T
Similar Instrument with
Model 42-31H Detector

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All of the features described above may be programmed manually using the
internal switch board or by computer through the RS-232 port. Two
different detector operating parameters may be stored in non-volatile
memory. The switch board can be removed after entering or changing
parameters to prevent tampering with setup parameters.
A regulated high-voltage power supply and independent set-point control,
adjustable from 400 to 2400 volts, with detector overload detection and
adjustable discrimination levels add versatility to the instrument. All of the
calibration controls are covered to prevent any inadvertent adjustments to
the detector operating parameters.
The instrument is powered by two standard ˝D˝cell batteries. The unit body
is made of cast-and-drawn aluminum with beige powder coating, which aids
in the decontamination of surfaces.
The attached Ludlum Model 42-31H Neutron Detector is designed for
detection of thermal and fast neutrons (0.025 eV to approximately 12
MeV). The neutrons are detected, not directly, but through nuclear
reactions, which result in energetically charged particles, such as alpha
particles. In many instances intense fields of gamma rays are also found
with neutrons. Therefore, it is important to choose a method of neutron
detection with the ability to discriminate against these gamma rays in the
detection process.
A common reaction for the conversion of slow neutrons into directly
detectable particles is n+3He→3H+1H + 0.764 MeV.
The Ludlum Model 42-31H utilizes this reaction in the form of helium-
3(3He), which fills the gas proportional tube of the detector
The Model 42-31H is designed to be used with portable counting
instruments and has a top bracket that allows for convenient mounting of
a portable instrument. The Model 42-31H consists of a 3He detector (1.6
cm diameter x 2.5 cm thick), surrounded by a cadmium-loaded
polyethylene sphere, 22.9 cm (9 inches) in diameter. A study is available
that shows that the 9-inch cadmium-loaded sphere has a response similar
to that of a 10-inch diameter rem-responding sphere.

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Getting Started
Unpacking and Repacking
Remove the calibration certificates and place them in a secure location.
Remove the instrument, detector, 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 2241-4 serial number is located on the front panel below the battery
compartment. To access the Model 42-31H detector identification, perform
the following: (1) loosen the thumbscrews; (2) rotate counterclockwise and
remove the polyurethane sleeve from the sphere; and (3) pull the detector
from sleeve. The identification is located on the detector shaft. Reassemble
in reverse order. Removal of the air-seal screw from the end of the
polyurethane sleeve will facilitate reassembly. Replace the screw once the
detector is completely reassembled.
Important!
If multiple shipments are received, ensure that the detectors
and instruments are not interchanged. Each instrument is
calibrated to a specific detector(s), and is 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. Include detector(s) and related
cable(s) for calibration.
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.
Section
2

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Battery Installation
Ensure the OFF/SCALER/RATEMETER switch is in the OFF position. Open
the battery lid by turning the quarter-turn thumb screw counterclockwise.
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.
Note:
The center post of flashlight battery is positive. The batteries
are placed in the battery compartment in opposite directions.
Detector Connection
Caution!
The detector operating voltage (HV) is supplied to the detector
via the detector input connector. A mild electric shock may
occur if bodily contact is made with the center pin of the input
connector. Switch the instrument OFF before connecting or
disconnecting the cable or detector.
Using the cable provided, connect the Model 42-31H detector to the Model
2241-4; firmly pushing the connectors together while twisting clockwise until
the connector latches (a quarter turn) as illustrated in the diagram to the left.
Operational Check
Note:
Performance of an operational check is an assurance of proper
instrument and detector function. The check should be
performed after instrument calibration or maintenance, as well
as prior to each use.
Turn the OFF/SCALER/RATEMETER switch to the RATEMETER position.
Notice that the display goes through an initialization sequence. The display

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will show all “8”s with decimal points. Ensure all segments display
as illustrated in the diagram to the left.
The LCD then displays the firmware number in the format
“P-XX YY.” The “XX” is the firmware number, and the “YY” is
the firmware version. (The figure to the left is for example only; to
illustrate location of display.)
The minimum displayable value (for example, 00.0 µR/hr) should be shown.
When switched to the SCALER position, a single “0” will be displayed.
The display will auto-range to the current level (see figure at left).
When auto-ranging down, the Model 2241-4 uses multiples of 5.
This technique keeps the decimal point from jumping between
numbers when viewing values around multiples of 10.
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.
Switch the AUD ON/OFF switch to the ON position and confirm that the
external unimorph speaker produces an audible click for each event detected
(audio divide-by 1 parameter). The AUD ON/OFF switch will silence the
clicks if in the OFF position; however, an audible alarm condition will still be
heard.
Move the OFF/SCALER/RATEMETER switch to the SCALER position. Depress
the COUNT switch located in the end of the carrying handle in order to
initiate a count cycle. The word “COUNTING”should be flashing on the
LCD during the count cycle and should disappear at the end of the
predetermined count time. If a scaler ALARM condition occurs, the RESET
switch can be depressed to acknowledge the alarm; however, the COUNT
switch must be depressed to clear the visual ALARM and to restart the count
cycle.
Depress and release the LIGHT switch. The backlight located behind the
LCD should illuminate (for pre-programmed ON time). Select the desired
F/S, AUD ON/OFF, and RATEMETER or SCALER parameters and proceed to
use the instrument.

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Specifications
0
Warm-up Time: Unit may be used immediately after the LCD
initialization sequence is completed (approximately five seconds after
power-up).
Linearity: Readings are within 10% of true value with a detector
connected.
Display: a four-digit Liquid Crystal Display (LCD) with digits 1.3 cm
(0.5 in.) in height. Two additional 0.5 cm (0.2 in.) digits are used for the
overflow counter (SCALER mode) and exponential powers (parameter
setup). Enunciators are provided for display units, ALERT, ALARM, low
battery, detector OVERLOAD, counting OVERFLOW, and scaler
COUNTING.
RATEMETER: Depending upon how the instrument was calibrated, the
RATEMETER can display in either R/hr, Sv/h, cpm, or cps when the
control switch is in the RATEMETER position.
SCALER: activated by pushbutton in handle when the three-position
switch is in the SCALER position. Count time is adjustable.
Calibration Controls: accessible from the front of the instrument
(protective cover provided). These controls are preset at the factory or
calibration lab and should not be adjusted by field personnel.
Discriminator / Input Sensitivity: adjustable from 2 to 100
mV; negative pulse response
Overload: indicated by OVERLOAD on the display; adjust-
able
High Voltage: adjustable from 400-2500 Vdc; regulated
within 0.2% at 1000 Vdc; maximum load of 50 µA
RESET: a pushbutton for zeroing the display, acknowledging and/or
resetting the alarm
LIGHT: display backlight activated by pushbutton
Audio: built-in audio speaker (unimorph) with AUD ON/OFF switch;
greater than 60 dB at 0.6 m (2 ft)
Alert/Alarm: indicated by either an ALERT or ALARM enunciator on the
display (RATEMETER mode only) and by an audible tone
Power: two˝D˝cell batteries housed in an externally accessible sealed
compartment. Current draw is approximately 35 mA with the backlight
OFF. Minimum battery voltage is 2.2 ±0.1 Vdc.
Section
3
Instrument:

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Battery Dependence: Meter readings vary by less than 3% from fully
charged batteries until the battery symbol appears, indicating the need
for recharge or replacement.
Battery Life: typically 200 hours with alkaline batteries (display indicates
low battery condition). Instrument will operate for approximately 24
hours after the battery symbol first appears.
Size: 16.6 x 8.9 x 21.6 cm (6. 5 x 3.5 x 8.5 in.) (H x W x D)
Weight: 1.6 kg (3.5 lb), including batteries
Backlight ON Time: 5, 15, 30, 60, 90, 120, 180, or 240 seconds for the
backlight to stay on when activated by the pushbutton; factory set at 5
Set Minimum Display: allows lower limit of the auto-ranging display to
be fixed. For example, the display can be set to only show values above
or equal to 1 µR/hr.
RS-232 Data Dump Mode: enables or disables dump mode to the RS-
232 port (˝D˝type connector). When enabled, the data will be dumped
every two seconds.
RS-232 Detector Setup Mode: allows for input of detector parameters
via the RS-232 port
Baud Rate: selects either 150, 300, 600, 1200, 2400, 4800, 9600, or
19200 bps
Detector Dead Time Compensation (DTC): adjustable from 0 to 9999
microseconds
Calibration Constant: adjustable from 0.001 to 280 × 109
counts/display unit
Display Units: can display in R/hr (representing rem/hr in this
application), Sv/h, cpm, or cps
Time Base: can display in seconds or minutes
Audio Divide: 1, 10, 100, or 1000 events per click
Response Time: variable or fixed ratemeter response (All stated times
correspond to a range of 10% to 90% of the final reading). The factory
default is ˝variable˝ so that the instrument will automatically adjust the
response time to the best setting for the current count rate.
Variable Response: dependant on the number of counts
present, typically 4 to 25 seconds for FAST and 4 to 60
seconds for SLOW.
Fixed Response: The FAST response position is adjustable
from approximately 2 to 50 seconds. The SLOW response
position is approximately five times slower than the FAST.
For MDA-type measurements, the fixed response mode is
recommended.
Removable
Switchboard
Adjustable
Parameters:

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Ratemeter Alert/Alarm: set at any point corresponding to the pre-
selected ratemeter range
Scaler Alarm: adjustable from 1 to 999999 counts
Scaler Count Time: adjustable from 1 to 9999 seconds
Detector: 3He proportional tube
Energy Range: thermal to approximately 12 MeV
Moderator: 22.9 cm (9 in.) diameter cadmium-loaded polyethylene
sphere
Sensitivity: typically 10 cpm per µSv/h (100 cpm per mrem/hr) (bare
AmBe neutrons)
Gamma Rejection: typically 10 cpm or less through 10 R/hr
(100mSv/h) (137Cs).
Energy Response: provides appropriate inverse RPG curve for
neutrons through 7 MeV, provides response up to 12 MeV
Input Sensitivity: -2 mV
Operating Voltage: approximately 1200 Vdc
Connector: series ˝C˝ (others available)
Size: 26.2 x 22.9 x 22.9 cm (10.3 x 9 x 9 in.) (H x W x D), including
brackets
Weight: 6.56 kg (14.5 lb)
Model 42-31H
Neutron Detector:

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Identification of Controls and
Functions
Display
The Model 2241-4 utilizes a four-digit liquid crystal display (LCD) with two-
digit overflow (SCALER mode) and moving decimal point. The two smaller
digits located in the lower right corner of the display indicate counter
OVERFLOW when in the scaler counting mode (equivalent to a six-
digit scaler) or exponential power when in the parameter setup mode.
The upper right corner of the LCD displays the following units and
multiplier(s): R/hr, mR/hr, or µR/hr; Sv/h, mSv/h, or µSv/h; C/m,
kC/m, C/s, or kC/s. The bottom part of the readout displays the
ALARM, ALERT, OFLOW, OVERLOAD annunciators and the low-battery
icon. COUNTING indicates that the scaler mode has been initiated and
is in the counting process.
Display Status Definitions
ALARM: Ratemeter or scaler count has increased above the preset alarm
threshold. An audible continuous tone will accompany the latching
ALARM condition. Depressing RESET will acknowledge the audible
ratemeter and/or scaler alarm. Depressing RESET a second time will
reset the ratemeter reading and ratemeter alarm. To reset the scaler
ALARM, depress the COUNT switch located in the carrying handle to re-
initiate the scaler count cycle.
ALERT: Ratemeter count has increased above the preset alert threshold.
To reset an ALERT condition, press RESET once if in the non-alarm
condition, and twice if in an alarm condition. (The first depression in
the alarm condition acknowledges the audible alarm.) The ratemeter will
reset to the minimum displayable reading each time the alert is reset.
OFLOW (Overflow) [RATEMETER mode]: indicates that the incoming
count exceeds the capability to display stable or reliable readings
corresponding to the radiation level being measured. The overflow
symbol will appear when the ratemeter exceeds 100 kcps or if the dead
time correction is greater than 75%. OFLOW will appear in the SCALER
Section
4

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mode when the six-digit display (four digits display and two overflow
digits in the right corner) reaches 999999 and starts to roll over again.
OVERLOAD: indicates that the detector is being exposed to radiation
intensities greater than the detector maximum operating limit. For alpha
and/or beta-type scintillation detectors, an OVERLOAD may indicate that
the detector face has been punctured allowing external light to saturate
the photomultiplier tube inside the detector. The overload alarm point
is set by adjusting the OVL control located underneath the calibration
cover.
Low battery icon:indicates that the batteries have decreased to the
minimum operating voltage of 2.2 ±0.1 Vdc. Instrument will continue
to operate for approximately 24 hours thereafter
COUNTING:indicates that the scaler count switch has been depressed
and that the scaler is accumulating counts for the pre-determined count
time
Front Panel Controls
OFF/RATEMETER/SCALER Switch: a three-position rotary switch that
applies power to the instrument and selects RATEMETER or SCALER
counting mode
AUD ON/OFF Switch: The clicks-per-event audio may be silenced or
enabled via this front-panel toggle switch. The audible alarm is
independent of the AUD ON/OFF switch and will override the audible
clicks-per-event. An audible alarm can only be silenced by depressing
the RESET button.
F/S (Fast/Slow) Response Switch:a two-position toggle switch that
selects fast or slow counting response time
Variable Response:The Fposition allows the time constant (TC) to
vary from 1 to 10 seconds, while the Sposition varies from 1 to 30
seconds. The response time is automatically adjusted in proportion to
the incoming count rate between the F/STC variables.
Fixed Response:The Fposition corresponds to the selected fixed
response time - TC. The Sposition is five times slower than the selected
fast TC.

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LIGHT (LCD Backlight):A pushbutton switch, when depressed,
illuminates the LCD for a pre-programmed time. The backlight ON time
can be selected between 5 and 240 seconds during the parameter setup.
RESET Pushbutton Switch:In the non-alarm condition, depressing the
RESET switch resets the ratemeter display to the minimum display
readout. In an alarm condition (ratemeter or scaler), depressing RESET
will silence the audible alarm. Depressing RESET a second time will reset
the ratemeter alarm and/or alert condition. The scaler alarm can only be
reset by depressing the scaler COUNT switch located in the end of the
Model 2241-4 handle.
Scaler COUNT Switch:pushbutton switch located in the end of the
Model 2241-4 carrying handle, which when depressed, initializes the
start of the scaler count accumulation for the preset scaling time. The
SCALER/RATEMETER switch must be in the SCALER position to initiate
the counting cycle. The scaler display uses the two digits in the lower
right-hand corner for the two most significant digits of the six-digit
readout. Scaling time can be set from 1 to 9999 seconds in the parameter
setup by way of the switch board. Depressing the COUNT switch after a
scaler ALARM will reset the scaler display to 0, resetting the alarm
condition.
Front Panel Calibration Controls
Note:
Remove the front-panel calibration cover to expose the
following calibration controls:
DISC (Discriminator): a multi-turn potentiometer (approximately 20
revolutions) used to vary the detector pulse-counting threshold from 2
to 100 millivolts. A Ludlum Model 500 Pulser or equivalent should be
used in checking or adjusting the pulse discrimination parameter.

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Note:
When making adjustments to the HV potentiometer, use a
Ludlum Model 500 Pulser or high-impedance voltmeter with a
high-voltage probe to measure the high voltage at the detector
connector. If a Ludlum Model 500 Pulser is not available,
ensure that the impedance of voltmeter used is 1000 megohms
or greater.
HV: a multi-turn potentiometer (approximately 20 revolutions) that
varies the detector voltage from 200 to 2500 volts. The maximum high-
voltage output is adjusted by the HV LIMIT potentiometer located on the
internal main board.
OVL (Detector Overload): a multi-turn potentiometer (approximately 20
revolutions), which adjusts the detector current level that must be
exceeded to initiate an OVERLOAD alarm. This control adjusts the
current level discrimination point from 0.5 and 40 microamperes,
corresponding to the specific detector saturation point.
Main Board Controls
Note:
To access the internal circuit boards, unlatch the latches at
each end of the Model 2241-4. Carefully separate the top
chassis from the bottom cover (referred to as a can). The can
has the audio speaker (unimorph) with a two-conductor cable
attached to the main board. The audio plug may be
disconnected during the internal control adjustments.
HV LIMIT (R027): A multi-turn potentiometer (approximately 20
revolutions) sets the maximum HV limit with the front-panel HV control
adjusted to the maximum clockwise position. It is adjustable from 1250
to 2400 Vdc.
VOLUME (R002): A multi-turn potentiometer (approximately 20
revolutions) varies the audible click-per-event and alarm audio. Adjust
the control to the maximum clockwise position for maximum volume.
If the VOLUME control is adjusted to the maximum counterclockwise

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position, the clicks-per-event or the audible alarm(s) will not be audible
when active.
Switch Board Controls
The switch board utilizes a 16-position rotary switch (FUNCTION) to
select the 16 setup parameters. (Refer to schematics and component
layout drawing near the end of the manual.) All of the setup parameters
are stored in the non-volatile EEPROM, which will retain data even
after the Model 2241-4 batteries are removed. After the parameters are
entered, the switch board can be removed and the Model 2241-4 will
continue to operate from the previously programmed information.
Changing parameters and information on switchboard controls are
covered in detail in Section 8 of this manual.