PiNSTECH-177

CALIBRATION OF NEUTRON SURVEY METERS

IFTIKHAR HUSSAIN

S. SALMAN AHMAD

KHALID MAHMOOD

IKRAM ULLAH QAZI

S. D. ORFI

HEALTH PHYSICS DIVISION Pakistan Institute of Nuclear Science and Technology

P. 0. Nilore, Islamabad August 2002

CONTENTS

S. No. DESCRIPTION PAGE

ABSTRACT

L INTRODUCTION I

2. THE NEUTRON IRRADIATOR 2

3. MATERIAL AND METHODS 4

3.1 CALIBRATION ARRANGEMENTS AND PROCEDURE 5

4. RESULTS AND DISCUSSION 5

5- CONCLUSION 7

ACKNOWLEDGEMENT 7

FIGURED 8

FIGURE-2 9

FIGURE-3 10

TABLE-1 U

TABLE-2 1 [

ANNEXURE-I 12

ANNEXURE-II 13

REFERENCES 14

ABSTRACT

With the financial and technicaJ support of International Atomic Energy

Agency (IAEA) a neutron survey instruments calibration facility has been

established at Secondary Standard Dosimetry Laboratory (SSDL) of Health Physics

Division, PINSTECH. This report presents the results of the neutron survey meters

calibration, carried out by PINSTECH for M/S CHINA ZHONGYUAN

ENGINEERING CORPORATION (CZEC), a CHINESE firm that has installed 300

MW nuclear power reactor at CHASHMA. These calibrated neutron survey meters

have been installed at CHASNUPP,

I. INTRODUCTION:

Neutron sources are, increasingly being applied in various fields such as research,

nuclear power development, industrial process, radiation biology and medicine. Calibration

of neutron survey meters is important to achieve meaningful results. Majority of the existing

SSDLs were established primarily to work with photons (Gamma rays and X-rays). The

SSDLs in countries where use of neutron sources is practiced on regular basis should also

fulfill the minimum requirement to calibrate neutron measuring devices [1],

Neutron dosimetry is a subject of much concern due to the complex nature. Neutrons

are uncharged and cannot cause ionization directly therefore, they behave differently from

charged particles and photons when passing through matter. Neutrons do not loose energy

either by deceleration as a sequence of direct ionization or by deceleration through radiation

emission. Their Interaction with orbital atomic electrons is so weak and it can be neglected.

There are different categories of standard sources - primary, secondary, laboratory

and operational. A primary standard source is a source that has been calibrated using a

method of absolute measurements and certified at a national laboratory in terms of

fundamental units. Such standards are subject to international intercomparison. When a

source is calibrated with reference to a primary standard, it is called a secondary source.

Calibrated sources should satisfy some special requirements. For gamma and neutron

sources the main requirement is angular uniformity of the gamma or neutron emission.

Isotopic gamma sources are calibrated in terms of exposure rate at a certain distance from

the source and isotopic neutron sources are calibrated in terms of the number of neutrons

emitted per unit time. Calibration of sources in a secondary standard laboratory is mainly

carried out by the method of relative measurements of the quantity under question.

A sealed neutron source suitable for calibration purpose should ideally have the

following qualities;

i) The source should have a reasonably long half-life;

ii) The physical size of the source should be small;

ill) The neutron emission rate should be high^

iv) It should have a known energy spectrum and should emit neutrons of a discrete

energy;

v) Tlic source should not emit radiation other than neutrons and the angular distribution

of the emission rate should be known. The emission rate and spectrum should also not be

affected by radioactive growth in the source by Ihe changes in d*e physical mixing of the

source.

There is no source that will possess all the above-mentioned qualities. When a

properly calibrated neutron source cannot be purchased, the neutron emission rates from

these sources should be absolutely determined. The most widely used method for this

determination is the manganese bath technique. Precision long counters are often used for

the absolute measurements of die fast neutron emission rates [2].

Some of the problems associated with the calibration of instruments designed for

Gamma ray and X-ray measurements are also applicable to neutron instrument. Neutrons

come in all energies {from 0.025 eV to more than 20 MeV) and the energy is the single

parameter, which complicates neutron monitoring. For an instrument designed to measure

neutrons over a wide energy range, a complete energy response determination must be

made. Knowledge of this will enable the instrument to be adjusted at the calibration energy

so that its overall accuracy at the other energies is improved.

As there existed no facility for the calibration of neutron survey instruments in the

country, a proposal was submitted to IAEA, to help Pakistan to acquire such a facility for

the calibration of neutron survey instruments at Secondary Standard Dosimetry Laboratory,

Health Physics Division, PINSTECH. The neutron Irradiator and related measuring facilities

were installed and commissioned in October-November, 1997.

2. THE NEUTRON IRRADIATOR:

The neutron irradiator (calibrator) model 149 serial number 12035 (FIGURES: 1-3)

was imported under the Technical Co-operation Project with International Atomic Energy

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Agency (IAEA). The irradiator is of United States of America origin and is manufactured by

M/S JL Shepherd & Associates, San Fernando, California, USA. The irradiator consists of a

single neutron source of 24JAm-Be. The activity of the source was 185 GBq (5 Ci) and the

neutron dose rate was 112 u.Sv/h atone meter m open beam condition on January 10, 1997r

A0.2TBq (5 Ci) Am-Be source will have a neutron output (emission rate) of (1,1-1.4) xJO7

neutron per second [I].

Due to nonavailability of separate bunker, the irradiator was installed in the X-ray

bunker in Secondary Standard Dosimetry Laboratory. The irradiator consists of two major

parts

1) Source casing

2) Control panel

The source casing is further divided into three parts. They are:

i) Base

ii) Source shield and

iii) Operational tower assembly

Tlie operating tower contains the pneumatic assembly (compressor etc.). The source

is operated by pneumatic cylinder controlled by a solenoid valve. The control panel in turn

activates this valve. All lines between solenoid valves and cylinders are equipped with flow

control valves. Automatic spring return assemblies are mounted on the cylinder. Air supply

of 60 PSI or greater is required to the inlet of the air filter regulator. The source is

mechanically returned by a constant force spring to the fully shielded position in the event of

loss of air pressure.

The source is operated when all the interlock lights (door interlock etc.) on the

control panel are illuminated and IRRADIATE button is pressed- The alarm will sound in

the room for preset time prior to source exposure. At the end of alarm interval, the source

will be raised to the EXPOSED position. The source may be returned to the fully shielded

position by opening or interrupting any interlock circuit OR end of the preset time selected

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OR pushing the OFF button OR pushing any interlock switch OR power failure OR loss of

air pressure.

The IRRADIATION light on the control panel is illuminated only when the source is

in the fully exposed position. The OFF light on the control panel is illuminated only when

ihe source is in the fully shielded position. The interlock light is illuminated only when all

interlocks are closed. Low -pressure light is illuminated when pressure falls below (evete set

on pressure switch. Source returns to the fully shielded position by air pressure whenever:

Any interlock switch is open OR OFF switch is operated OR timer times out OR power

failure.

The source is mechanically returned by a. constant force spring to the fully shielded

position in the event of loss of air pressure. The shield design provides for full shielding in

alt directions, at all times, except out the beam port when the source is in the ON position.

The spring return assembly on the pneumatic source operating cylinder provides fail-

save operation, in that source automatically returns to the OFF position in case of air or

power failure. In addition, the normally closed solenoid valve provides pressure to hold the

source in the OFF position whenever air pressure is supplied to the system and the ON

switch is not activated.

The source rod never touches the bottom of the tube in which it travels, clearance is

approx. 0.5 inch, eliminating the possibility of damage to the source by striking the end of

the source tube [3],

3. MATERIAL AND METHODS:

The irradiator (calibrator) was installed and made operational. The optimization of

the calibration procedure for neutron survey meters was carried out and the sample

calibration certificate was prepared.

Neutron survey meters of M/s China Zhongyuan Engineering Corporation (CZEC),

an international firm that installed 300 MW nuclear power reactor at Chashma, Kundian,

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district Mianwafi were calibrated for the first time in the Secondary Standard Dosimetry

Laboratory after the installation of the neutron calibrator. The Dose measurement data and

the calibration results of the neutron survey meters are presented in the Tables 1-2.

3.1.CAHBRATION ARRANGEMENTS AND PROCEDURE:

The neutron survey meter to be calibrated was aligned along the central beam axis of

the neutron irradiator at the distance of one meter from the center of the neutron source.

Field size of the neutron beam at one meter was sufficiently larger (approx. 50 cm diameter)

than the maximum size of the neutron survey meter detector. A slight modification was

made in order to switch ON and OFF the survey meter under safe conditions out side the x-

ray bunker. It was because of that modification that the survey meter could not be operated

during the irradiation measurements.

Preliminary operational checks including battery check were performed. The

background radiation was measured after alignment along the central neutron beam axis.

The irradiator was turned ON from the control panel, which was placed in the control

room out side the x-ray bunker. The calibration measurements were carried out under (i) The

OPEN beam condition (Figure-1) and (ii) COLLIMATED beam condition (Figure- 2). For

each set of measurements five readings were carried out.

In order to view and safely carrying out the readings in the control room out side the

x-ray bunker, a CCTV camera was focused on the survey meter scale.

The two survey meters that were calibrated and have been discussed in this report are

portable neutron dose-equivalent meter model BH 3105, Sr. No. 02 & 03 [4].

4. RESULTS AND DISCUSSION:

The calibration of both the neutron survey meters was carried out in the same

standard calibration conditions and geometry i.e. in the Open beam (Fig.-!) geometry and

the Colli mated (Fig.-2) beam geometry. In the Open beam geometry the field size is larger

resulting in the availability of slightly higher radiation field with broader spectrum of

neutron energy. In the Collimated beam geometry the field size is smaller compared with

the open field geometry resulting in the availability of low radiation field with narrow

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spectrum of neutron energy. Therefore, two beam geometries are available for [he

calibration. In both the beam geometry condition (Open and Collimated) the field size was

sufficiently larger than the siae of the detector of neutron survey meter.

The neutron dose measurement data and the calibration results of the first survey

meter (model BH 3105 Sr. No. 02) are given in Table-1.

In the Open beam (Fig.-l) condition the survey meter read 155.26 uSv/h. The

reference field at one meter in the open beam condition was 112.0 uSv/h So the meter over

estimated the dose rate. The calibration factor came to be 0,721.

In the Co) lima ted beam (Fig.-2) condition the meter read HT32 jxSWh. The

reference field at one meter in the Collimated beam condition was 105.0 u-Sv/h. Again the

meter over estimated the dose rate and the calibration factor came to be 0.895 in this

condition.

The calibration results of the second Survey meter (model BH 3105 Sr. No. 03) are

given in Table-2,

In the Open beam condition (Fig.-1) the meter read 148.46 |xSv/h The reference field

at one meter in the open beam condition was 112.0 \xSvfh The meter over estimated the dose

rate and the calibration factor came to be 0.754.

In the Collimated beam condition (Fig.-l) the meter read 113.74 jiSv/h. The

reference field at one meter in collimated beam condition was 105.0 iiSv/h. Again the

survey meter over estimated the dose rate. The calibration factor came to be 0.923. As

already mentioned the standard calibration conditions (beam geometry, distance etc.) and

alignment procedure was kept same during the calibration of both (he survey meters.

Both the survey meters (model BH 3105, Sr. Nos. 02, 03) over estimated the dose

rate in the Open beam condition and in the Collimated beam condition. However, the

percentage error is less in the Collimated beam condition as compared with the Open beam

condition. The reason for the higher error in the open beam condition is that the field size in

the open beam condition is larger as compared with the collimated beam condition and

therefore, the scattering etc. is higher in the open beam condition as compared wilh the

co Miniated beam condition.

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For genera! radiation field survey calibration factor for the open beam geometry is

preferable, although the percentage error in this case will be higher compared with the

collimated beam geometry as discussed already and is also obvious from the data of

measured neutron dose given in the Tables I & 2.

The quantities used in the calibration tables & certificates are defined as follows:

Calibration Factor = Reference Field/Instrument Reading

% Error - [(Instrument Reading - Reference Ficld)]/Rcfcrcncc Field] x 100

l S v = 100 rem

5. CONCLUSION:

The CZEC technical representatives handed over the neutron survey meters to

PINSTECH, SSDL after complete assessment of our calibration facilities &. capabilities. The

neutron survey meters were calibrated and the calibration certificates, containing all the

details of calibration, were prepared and delivered to M/s China Zhongyuan Engineering

Corporation (CZEC) on time, as promised. Calibration Certificates are reproduced in the

ANNEXURES I & II.

The neutron survey instruments calibration facility of Secondary Standard Dosimetry

Laboratory, Health Physics Division, PINSTECH is fully operational and is being made

available to the end-users, in the country,

ACKNOWLEDGEMENT:

The authors arc grateful to Dr. Abdul GhafFar (SI), Director General, PINSTECH for

providing technical, administrative and financial support We also acknowledge the help

received during calibration of neutron survey meters from all the members of HPD in

general and the technical staff of the Secondary Standard Dosimetry Laboratory in

particular. Financial help of IAEA, which enabled us to procure neutron irradiator, is also

gratefully acknowledged.

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FIG-1: Set-up for the calibration of neutron survey instrument in OPEN BEAM condition

S

Figurc-2: Sel-up for the calibration of neutron survey instruments in collimated Beam Condition

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Fig-3: Control Panel of Neutron Irradiator Model 149

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TABLE 1: Neutron Dose Measured With Neutron Survey Meter Model BH 3105 S, No. 02

ST.

I

2

Distance

(cm)

100

100

Benin Condition

Open Beam Port

Wiih Collimator

Measuring Range

(liSvA)

01-1,00,000

-do-

i

Rcferenct Field

(nSv/li)

112.0113%

105.0 ± 1 3 %

Average Instrument Reading

OiSvfli)

155.26

tor,., - ± 0.42

117,32

MaBll - ± 0.57

Error

%

33.62

11.73

Calibration Factor

0.721

0.895

TABLE-2: Neutron Dose Measured With Neutron Survey Meter Model BH3I05 S. No. 03

5r. No.

1

2

Distinct

(cm)

100

I0E)

Beam Condition

Open Beam Port

With Collimator

Measuring Range

OiSv/h)

0.1-1,00.000

-do-

Reference Field

(nSv/h)

112.0 ± 1 3 %

105.0 ± 1 3 %

Average Instrument Reading

tySvm

148.46

%o„., - ± 1.11

113.74

%B„.1=±I.04

Error

%

32.55

8.32

Calibration Factor

0.754

0,923

Calibration Factor = Reference Field/Instrument Reading

% Error - [(Instrument Reading - Reference Field)]/Reference Field] x LOO

I Sv - 100 rem

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SAMPLE

ANNEXURE-1: First Neutron calibration certificate issued to CZEC

CHASHMA NUCLEAR POWER PLANT, KUNDlAN (April, 1999)

SECONDARY STANDARD DOSIMETRY LABORATORY

HEALTH PHYSICS DIVISION PAKISTAN INSTITUTE OF NUCLEAR SCIENCE AND TECHNOLOGY

CALIBRATION CERTIFICATE OF NEUTRON SURVEY METER

Certificate No.: - NRP-01/99 Date of issue: - 05-04-1999

Customer: - CZEC-CHASNUPP

Instrument Type & Serial No.:- BH 3105,& 02(BNIF)

Neutron Source Used :- ^'Am-Be^SCi)

Reference Output:~ 11,2 mrem7h±l3% in "Open beam port" and 10.5 raremfli ±13% "With Collimator" at a distance of one meter from (he center of the source. Reference to certificate of manufacturer dated January 10, 1997 traceable to N1ST. Report No. 256551 or Jnne 14, 1996.

Next Calibration due:- April, 2000

Calibration Conditions (Geometry )> Source to Detector Distance 1QQ cm

SrNo.

1

2

Distance

(cm)

100

100

Beam Position

Open Beam Port

With Collimator

Measuring Range

(uSv/h)

0J-1,00,000

-: do :-

Reference Field

(uSv/h)

112.00* 13%

L05.OO±I3S

Average Instrument Reading (uSvyn)

155.26

11732

Error

%

38.62

IL73

Calibration Faclor

0.721

0.89S

Calibration Factor - Reference Field / Instrument Reading % Error = [(Instrument Reading - Reference Field J / Reference Field J x 100 I Sv = 100 rem

Remarks :- Overall uncertainties arc related. to die conditions of die determination of til? calibration in the beam of Am-Be as stated above- If the instrument is employed al other energies different from above, additional uncertainties may become applicable as related to the instrument energy dependence given in the corresponding manual. Methods and results of calibration may contribute another additional uncertainty of 4 10%.

Calibrated and Prepared by Checked by

Counter Signed

HEAD HEALTH PHYSICS DIVISION

Head SSDL

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SAMPLE

ANNEXURE-Il: Second Neutron calibration certificate issued to CZEC

CHASHMA NUCLEAR POWER PLANT, KUNDIAN (April, 1999)

SECONDARY STANDARD DOSIMETRY LABORATORY

HEALTH PHYSICS DIVISION PAKISTAN INSTITUTE OF NUCLEAR SCIENCE AND TECHNOLOGY

CALIBRATION CERTIFICATE OF NEUTRON SURVEY METER

Certificate No.; - NRP-Q2/99 Dale of Issue: -I2-W-I999

Customer: - CZEC-CHASNUPP

Instrument Type &. Serial No.; - EH 3105, & 03(BNIF)

Neutron Source Used: - " 'Am-B^SCO

Reference Output: - 11.2 mrem/h±l3% in "Open beam port" and 10.5 mrera/h ±I3V. "With Collimator* at a distance of one meter from tne center of the source. Reference lo certificate of •ainufacturer dated January 10, 1997 traceable (o NIST. Report No, 256551 of June 14,1996.

Next Calibration due: Apri l 2000

Calibration Conditions (Geometry): - Source to Detector Distance 100 em

SrNo,

1

2

Distance

(cm)

ICO

100

Beam Position

Open Beam Port

With Collimator

Measuring Range

(uSv/h)

0,1-1,00,000

-: do :-

Reference Field

(uSvyh)

112.00 ±13%

105.00 ± 13%

Average Instrument Reading (jiSvrti)

14&46

113.74

Error

%

32.55

S,3Z

Calibration Factor

0.7S4

OJJI3

Calibration Factor = Reference Field / Instrument Reading % Error = [(Instrument Reading - Reference Field) 1 Reference Field] x 100 JSY-lOOrem

Remarks:- Overall uncertainties are related to the conditions of the determination of the calibration in the beam of MlAm-Be as stated above. I f the instrument is employed at other energies different from above, additional uncertainties may become applicable as related to the instrument energy dependence given in the corresponding manual. Methods and results of calibration may Contribute another additional uncertainly of ± 10 %.

Calibrated and Prepared by Checked by Head SSDL

Counter Signed

HEAD HEALTH PHYSICS DIVISION

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REFERENCES

[1] Guidelines On Calibration Of Neutron Measuring Devices,

Technical Report Series No, 285

IAEA, Vienna, Austria (1985).

[2] Handbook On Calibration Of Radiation Protection Monitoring Instruments

Technical Report Scries No. 133

IAEA, Vienna, Austria (1971).

[3] Installation & Operation Manual For Model 149

Single Source Neutron Irradiator,

San Fernando, California, USA (1997).

[4] Portable Neutron Dose-Equivalent Meter Model BH 3105

Operating & Service Manual

Beijing Nuclear Instrument Factory

Beijing CHINA.

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