LUDLUM MODEL 42-49
NEUTRON DETECTOR
December 2017
LUDLUM MODEL 42-49
NEUTRON DETECTOR
December 2017
Model 42-49 Neutron Detector Technical Manual
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 Technical Manual Section 1
Section
1
T
Introduction
he Ludlum Model 42-49 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 42-49 utilizes this reaction in the form of helium-3(3He),
which fills the gas proportional tube of the detector
The Model 42-49 is 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.
Ludlum Measurements, Inc. Page 1-1 December 2017
Model 42-49 Detector shown as part of a Model 30-7
Model 42-49 Neutron Detector Technical Manual Section 1
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.
Ludlum Measurements, Inc. Page 1-2 December 2017
Model 42-49 Neutron Detector Technical Manual Section 2
Section
2
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.
Ludlum Measurements, Inc. Page 2-1 December 2017
Model 42-49 Neutron Detector Technical Manual Section 3
Section
3
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: 100 cpm/mrem/hr (
Gamma Rejection: 10 cpm or less through 10 R/hr (100 mSv/hr) (
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)
241
AmBe fast neutrons)
137
Cs)
Ludlum Measurements, Inc. Page 3-1 December 2017
Model 42-49 Neutron Detector Technical Manual Section 4
Section
4
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.
Ludlum Measurements, Inc. Page 4-1 December 2017
Model 42-49 Neutron Detector Technical Manual Section 5
RateEquivalentDose
RateCount
ySensitivit
Section
5
Calibration
Calibration
To calibrate the detector, the following items are needed:
Counting instrument: Ludlum Model 2200 Scaler/Ratemeter
241
AmBe neutron source or equivalent
137
Cs 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
“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:
241
AmBe neutron source.
Ludlum Measurements, Inc. Page 5-1 December 2017
Model 42-49 Neutron Detector Technical Manual Section 5
hrmremcpm
hrmrem
cpm
ySensitivit
//119
/20
2380
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:
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
137
Cs 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.
Ludlum Measurements, Inc. Page 5-2 December 2017
Model 42-49 Neutron Detector Technical Manual Section 5
Ref. Point
(mrem/hr)
Reading
(cpm)
Range/
Scale
400
200
80
20
8
2
Conversion Chart
Expose the detector to an
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.
241
AmBe neutron source at a dose-
Table 1
Ludlum Measurements, Inc. Page 5-3 December 2017
Model 42-49 Neutron Detector Technical Manual Section 6
Section
6
Model 42-49
Neutron Detector
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
*
* 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
3
He TUBE 25185 2 ATM 01-5793
Ludlum Measurements, Inc. Page 6-1 December 2017
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