Ludlum Model 42-5 Neutron Ball Cart
September 2020
Serial Number PR149512 and Succeeding Serial Numbers
1. General
The Model 42-5 Neutron Detector consists of a 6LiI crystal coupled to a photomultiplier tube. The unit is designed so that the center of the crystal will be positioned in the center of the Model 42-5 moderator spheres. The detector is mounted on the Model 42-5 ball cart and secured with set screws.
The height of the detector is adjustable to allow the sphere to be positioned correctly.
For further information, see the report, Hankins, D. E., "A Method Of Determining The Intermediate Energy Neutron Dose," IDO-16655, March 10, 1961.
2. Specifications
DETECTOR: 4 mm x 4 mm 6LiI (Eu) crystal (lithium used is 96% enriched lithium), coupled to 3.8 cm (1.5 in) photomultiplier tube
TUBE: 1.2 cm (0.5 in.) diameter magnetically shielded photomultiplier
MODERATORS: high-density polyethylene spheres in the following diameters: 30.5 cm, 25.4 cm, 20.3 cm, 12.7 cm, 7.6 cm, 5.1 cm (12 in., 10 in., 8 in., 5 in., 3 in., 2 in.)
CART: tubular steel construction, provides a mobile storage unit for the moderators. The crank-operated elevator assembly adjusts the detector center from 112 cm (44 in.) to 185 cm (73 in.) from the floor.
CONNECTOR: series "C"
SENSITIVITY: typically 45 cpm/mrem/hr (AmBe) (with 25.4 cm [10 in.] moderator)
GAMMA REJECTION: 10 cpm or less through 100 mR/hr
ENERGY RESPONSE: continuous response from thermal to approximately 12 MeV
COMPATIBLE INSTRUMENTS: any instrument capable of detecting negative pulses and supplying high voltage of up to 1500 volts and a discrimination threshold of approximately 2 mV
OPERATING VOLTAGE: typically 400-900 volts
TEMPERATURE RANGE: -20 to 50 °C (-4 to 122 °F); may be certified to operate from -40 to 65 °C (-40 to 150 °F)
SIZE: 119 x 45.7 x 58.3 cm (44 x 18 x 23 in.) lowered (H x W x L); 185 cm (73 in.) in length fully extended
WEIGHT: 27.2 kg (60 lb), not including moderators
3. Operating Procedures
Before using the instrument, verify that the instrument has been calibrated according to Section 4.
CAUTION! DO NOT move the cart with a moderator sphere mounted on the detector. The moderator could fall off causing damage and/or injury.
- Move cart into the location that will be surveyed.
- Mount the detector into the top of the adjustable rod between the handles of the ball cart. Attach cable to instrument.
- Remove the desired moderator from its storage position and set on top of the detector.
- Turn the crank handle on the side of the cart to raise the detector into position. To lower the detector, press in on the locking lever on the lower rear of the gear box.
- Turn on the instrument and proceed to take readings.
- Varying the diameter of the spheres or using a different source will change the response of the unit. See chart in the back of this manual.
4. Calibration
The following calibration procedures assume the following:
- Counting instrument is Ludlum Model 2200 Scaler/Ratemeter
- 241AmBe neutron source (Note that the Model 42-5 is energy dependent.)
- 137Cs gamma source for gamma rejection check
4.1 Operating Voltage Plateau
Connect the Model 42-5 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 900 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 900-1300 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:
Sensitivity = Count Rate / Dose-Equivalent Rate
For example, an assumed operating voltage is 1050 volts, based upon the flattest part of the plateau. The count rate at that voltage is 1311 counts per minute (cpm), and the neutron field dose-equivalent rate is 20 mrem/hr. The sensitivity is calculated as:
Sensitivity = 1311 cpm / 20 mrem/hr = 65.55 cpm / mrem/hr
This value should be approximately 60 cpm/mrem/hr.
4.2 Gamma Rejection Check
With the Model 42-5 connected to the Model 2200, adjust the Model 2200 HV to the assumed operating voltage determined above. Remove the Model 42-5 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 60 cpm/mrem/hr. Return the Model 42-5 to the moderator.
4.3 Conversion Chart
Expose detector to 241AmBe neutron source at the dose-equivalent rate of 400 mrem/hr. Take a one-minute count and record value, including 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.
| Ref. Point (mrem/hr) | Reading (cpm) | Range/Scale |
|---|---|---|
| 400 | ||
| 200 | ||
| 80 | ||
| 20 | ||
| 8 |
5. Parts List
Model 42-5 Neutron Ball Cart
UNIT
| Ref. No. | Description | Part No. |
|---|---|---|
| Completely Assembled Model 42-5 Neutron ball cart | 47-1505 |
Detector Probe
| Ref. No. | Description | Part No. |
|---|---|---|
| 1 ea. | M42-5 Detector Probe | 47-1507 |
| 1 ea. | Crystal Holder Assembly | 7014-002 |
| 1 ea. | Crystal – 4 mm x 4 mm | 01-5131 |
| 1 ea. | Model 42-5 Crystal Spacer | 7014-538 |
| 1 ea. | Model 42-5 Detector Main Body | 7014-001 |
| 1 ea. | Model 42-5 End Plug | 7014-003 |
| 1 ea. | Model 42-5 Connector Ring | 7014-004 |
| 1 ea. | Model 42-5 Light Pipe | 7014-376 |
| 6 ea. | Model 44-3, 44-2, 43-5 Sponge | 7002-029-05 |
| 1 ea. | Tube/Skt Assy. | 4002-510 |
| 1 ea. | 0.5 inch Photomultiplier Tube | 01-5349 |
| 1 in. | Wire-Teflon Wht #22 HV | 21-8993 |
| 1 in. | Wire #26 Black 0 UL1429* | 21-9432 |
| 1 ea. | RECPT-UG706/U "C" LMI | 4478-011 |
| 1 ea. | Tube Shield | 40-4006 |
| 1 ea. | Lug-1497; 707-1774 | 18-8776 |
Ball Cart
| Ref. No. | Description | Part No. |
|---|---|---|
| [*] | Model 42-5 Less Detector | 47-1506 |
| 1 ea. | Model 42-5 Ball Cart | 47-1110 |
| 1 ea. | Model 42-5 2-inch Ball Holder | 7014-004-01 |
| 1 ea. | Model 42-5 3-inch Ball Holder | 7014-004-02 |
| 1 ea. | Model 42-5 5-inch Ball Holder | 7014-004-03 |
| 1 ea. | Model 42-5 8-inch Ball Holder | 7014-004-04 |
| 1 ea. | Model 42-5 10-inch Ball Mount | 7014-017-06 |
| 1 ea. | Model 42-5 12-inch Ball Mount | 7014-017-05 |
Spheres
| Ref. No. | Description | Part No. |
|---|---|---|
| 1 ea. | Model 42-5 2-inch Sphere | 7014-009-05 |
| 1 ea. | Model 42-5 3-inch Sphere | 7014-009-04 |
| 1 ea. | Model 42-5 5-inch Sphere | 7014-009-03 |
| 1 ea. | Model 42-5 8-inch Sphere | 7014-010-01 |
| 1 ea. | Model 42-5 10-inch Sphere | 7014-009-02 |
| 1 ea. | Model 42-5 12-inch Sphere | 7014-009-01 |
6. Drawings, Diagrams, Charts
The following technical drawings and charts are available:
- Model 42-5 Probe Assembly, Drawing 14 x 32A: This diagram illustrates the internal components and construction of the detector probe, including dimensions and revision history.
- Model 42-5 12-inch Sphere and Detector, Drawing 14 x 32B: This diagram shows the detector probe positioned within a 12-inch moderator sphere, detailing the overall assembly and dimensions.
- Tube Basing Diagram, Drawing 2 x 147: This schematic depicts the electrical connections and component layout for the photomultiplier tube socket.
- Relative Counting Rate vs Energy for Moderating Spheres, Chart: This graph plots the relative counting rate against neutron energy for various moderator sphere sizes (2", 3", 5", 8", 12").
