LUDLUM 42-49B User manual

LUDLUM MODEL 42-49 & 42-49B
NEUTRON DETECTOR
October 2021

LUDLUM MODEL 42-49 & 42-49B
NEUTRON DETECTOR
October 2021

Model42-49&42-49BNeutronDetector TechnicalManual
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]

Model42-49&42-49BNeutronDetector TechnicalManual
Table of Contents
Introduction 1
Unpacking and Repacking 2
Connecting to an Instrument 2-1
Specifications 3
Safety Considerations 4
Environmental Conditions for Normal Use 4-1
Cleaning Instructions and Precautions 4-1
Calibration 5
Calibration 5-1
Operating Voltage Plateau 5-1
Gamma Rejection Check 5-2
Conversion Chart 5-3
Parts List 6
Model 42-49 Neutron Detector 6-1
Model 42-49B Conversion Kit 6-2
Drawings and Diagrams 7

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Introduction
he Ludlum Model 42-49 and 42-49B Neutron Detectors are 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 42-49 and 42-49B utilize this reaction in the form of
helium-3(3He), which fills the gas proportional tube of the detector
The Model 42-49 and 42-49B are designed to be used with portable counting
instruments and has a top bracket that allows for convenient mounting of a
portable instrument. (See picture below.) The Model 42-49 consists of a 3He
detector (1.6 cm diameter x 2.5 cm thick), surrounded by a high density
polyethylene sphere, 19.6 cm (7.7 inches) in diameter. This outer sphere
moderates the incoming neutrons and helps provide a near-rem response.
Model 42-49 Detector shown as part of a Model 30-7
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1
T

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Note:
The detector does not contain any consumable materials.
Note:
If the detector is used in a manner not intended by the manufacturer,
the detector may not function properly.
The Model 42-49 was first introduced in 2017, and is similar to the 42-49B,
sharing the same 7.7 inch diameter ball. The Model 42-49 has better sensitivity
than the 42-49B, similar to the 9-inch remballs, and many customers are happy
with the energy response. But some customers noted differences in readings
between the Model 42-49 and the older Model 12-4 with its 9-inch ball. It was
discovered that the boron content of the inner shell of the 42-49 was not high
enough, causing too much of an over-response in the 5 keV region. It was also
noted that angular dependence, especially at 180 degrees, was excessive, due to
a lack of moderation within the detector entry shaft.
The 42-49B corrects these two issues by increasing the boron content of the
inner shell, and by adding a boron moderator and polyethylene spacer in the
detector entry shaft. These corrections to the energy response do have a
drawback –overall neutron sensitivity is approximately half that of the original
42-49. It is fairly simple (takes less than an hour) to replace these two parts and
convert the earlier 42-49 to a 42-49B. The part number of the Ludlum
conversion kit with these two parts is 4014-620-01. A recalibration is required
after this conversion is made.
Shown above are the two different boron moderators attached to the detector, which fit into a rem ball.
The one on the left is a 42-49, and the one on the right is a 42-49B.

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Unpacking and Repacking
Remove the calibration certificate or detector functional check
certificate and place it in a secure location. Remove the detector(s)
and accessories (if applicable) and ensure that all items listed on the
packing list are in the carton. If multiple detectors are included, refer
to the calibration certificates for serial number (SN) matches.
To return an instrument or detector 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 on 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.
Connecting to an Instrument
Connect one end of the cable provided to the detector by
firmly pushing the connector together while twisting
clockwise a quarter of a turn until latched. Repeat the
process in the same manner with the other end of the
cable and the instrument.
Section
2

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Specifications
0
Detector: 2 atm ³He tube LND 25185 or equivalent
Moderator: 19.6 cm (7.7 in.) diameter high density polyethylene sphere
Compatible Instruments: typically used with portable counting
instruments (capable of achieving -2 mV input sensitivity)
Sensitivity:
Model 42-49: 100 cpm/mrem/hr (241AmBe fast neutrons)
Model 42-49B: 45 cpm/mrem/hr (241AmBe fast neutrons)
Gamma Rejection: 10 cpm or less through 10 R/hr (100 mSv/hr) (137Cs)
Detection Range: thermal to approximately 12 MeV
Energy Response: thermal to 12 MeV, follows the radiation protection
guide curve for neutron dose
Input Sensitivity: -2 mV
Operating Voltage: approximately 1200 V
Connector: series “C” (others available)
Temperature Range: -20 to 50 °C (-4 to 122 °F)
Size: 25.5 x 19.6 x 19.6 cm (10 x 7.7 x 7.7 in.) (H x W x D), including
brackets
Weight:4.6 kg (10.2 lb)
Section
3

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Safety Considerations
Environmental Conditions for Normal Use
Indoor or outdoor use (in a dry environment)
No maximum altitude
Temperature range of -20 to 50 °C (-4 to 122 °F); may be certified for
operation at -40 to 65.6 °C (-40 to 150 °F)
Maximum relative humidity of less than 95% (non-condensing)
Pollution Degree 2 (as defined by IEC 664) (Normally only
nonconductive pollution occurs. Temporary conductivity caused by
condensation is to be expected.)
Cleaning Instructions and Precautions
The 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:
1. Turn the instrument electronics OFF.
2. Allow the instrument to sit for one minute.
3. Disconnect the detector cable before cleaning the detector.
Section
4

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Calibration
Calibration
To calibrate the detector, the following items are needed:
Counting instrument: Ludlum Model 2200 Scaler/Ratemeter
241AmBe neutron source or equivalent
137Cs gamma source for gamma rejection check
Operating Voltage Plateau
Connect the Model 42-49 to a Model 2200.
Set Model 2200 input sensitivity to -2 mV.
Expose the detector to a 20 mrem/hr 241AmBe neutron source.
“Run a plateau” as follows: Taking one-minute counts, record the
count rate for 25-volt incremental steps from 1000 volts through a
region in which the count rate steadily increases, becomes relatively
constant (“flattens out”), and then increases again. (A typical range
will be 1000-1250 volts.) The flattest portion of this region is the
desired plateau. The value in the middle of this region is the assumed
operating voltage of the detector, subject to the sensitivity and the
gamma rejection checks.
Calculate the sensitivity (cpm/mrem/hr) for the assumed operating
voltage as follows:
RateEquivalentDose RateCount
ySensitivit  
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For example, an assumed operating voltage is 1100 volts, based upon the
flattest part of the plateau. The count rate at that voltage is 2380 counts
per minute (cpm), and the neutron field dose-equivalent rate is 20 rem/hr.
The sensitivity is calculated as:
hrmremcpm
hrmrem
cpm
ySensitivit
//119
/20
2380


This value should be approximately 120 cpm/mrem/hr.
Gamma Rejection Check
With the Model 42-49 connected to the Model 2200, adjust the Model
2200 HV to the assumed operating voltage determined above.
Remove the Model 42-49 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 42-49 to the moderator.

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Conversion Chart
Expose the detector to an 241AmBe neutron source at a dose-
equivalent rate of 400 mrem/hr. Take a one-minute count and record
the value, including the range/scale setting of counting instrument.
Repeat for the dose-equivalent rates shown in Table 1.
The values in Table 1 and their corresponding measured values represent
a conversion chart for use in relating other measured values to actual
dose-equivalent rates.
Table 1
Ref. Point
(mrem/hr)
Reading
(cpm)
Range/
Scale
400
200
80
20
8
2

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Parts List
Reference Description Part Number
UNIT Completely Assembled
Model 42-49
Neutron Detector 47-4057
2 EA. SCREW-6-32 X 3/8 BRASS KNURL
17-8659
* Model 42-49 SPHERE ASSY 47-4058
4 EA. SCREW-#8 X ¾ FH SHT MTL 17-9085
* MODEL 43-4, 324 HANDLE 7005-060
* 7.7 inch SPHERE 7014-858
2 EA. INSERT E-Z LOK 6-32 20-9066
8 EA. 8 INCH HDPE ROD 29-9985
* MODEL 42-49 DET + MODERATOR
47-4059
1 EA. O RING-2-116 16-8293
1 EA. O RING-2-148 16-8340
2 EA. SCREW-8-32 X 5/8 SOC SET 17-8765
* MODEL 42-49 PROBE ASSY 4014-587
* 3He TUBE 25185 2 ATM 01-5793
* CONNECTOR, UG706/U 4478-011
* TUBE BODY ASSY 2014-595
* MODEL 42-30, 42-31
DETECTOR SPACER 7005-056
* CONNECTOR END 7014-598
* CBL-MODEL 42-49 RG178 3INCH
8014-599
* MODEL 42-49 MODERATOR 7014-591
* MODEL 42-49 DETECTOR STRAP
7014-592
Section
6
Model 42-49
Neutron Detector

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Reference Description Part Number
Model 42-49 Conversion Kit 4014-620-01
1 EA. Borated Silicone Shell 5/32 HL 7014-609
1 EA. Borated Silicone Shell, 3/4 HL 7014-610
1 EA. Borated Silicone Detector Disc 7014-621
1 EA. Detector HDPE Spacer Short 7014-622
2 EA. Borated Silicone Slot Filler 7014-625
Model 42-49B
Conversion Kit

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Drawings and Diagrams
Model 42-49 Overall, Drawing 14 x 584
Model 42-49 Probe Assembly, Drawing 14 x 587
Model 42-49B Probe Assembly, Drawing 14 x 620
Section
7