Tsoulfanidis N. Measurement and detection of radiation

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508

MEASUREMENT

AND

DETECTION

RADIATION

Table

14.5

Typical Values of

for Several Neutron

Energies

(eV)

t/L

bslm)

(MeV)

1/15

(nslm)

0.01

722

0.1

228

0.1

228.5

72.3

72.2 2 51.2

22.8 5 32.4

100

7.2

1000 2.3 20 16

tially, as

E~/~

(compare Eqs. 14.52 and 14.57):

14.8.1

The Neutron Velocity Selector (Neutron Chopper)

The first velocity selector was designed by Fermi and his co-workers in the 1940s

and is now known as the Fermi chopper.49 The Fermi chopper consisted of a

multiple sandwich of aluminum and cadmium foils that fit tightly into a steel

cylinder about

mm (1.5 in) in diameter (Fig. 14.22). The cylinder was rotated

at speeds of up to 15,000 r/min, thus allowing only bursts of neutrons to go

through the aluminum channels. Based on the geometry of Fig. 14.22, no

neutrons from a parallel beam would go through the channel when the chopper

was more than A8/2 degrees from its fully open position (Fig. 14.23), where

(width of channel)/(radius of cylinder). The spinning cylinder was viewed

with two photocells, one giving a direct measure of the rotation and the other

sending to the neutron detector a signal used for the measurement of the

time-of-flight of the transmitted neutrons.

Incident,

beam

Steel

ging beam

Figure

14.22

The

Fermi chopper.

NEUTRON

DETECTlON

AND

SPECTROSCOPY

509

Incident

beam

Figure 14.23

(a)

The channel is fully open.

(b)

The chopper has rotated

AO/2,

and the channel is

closed.

Fermi's chopper was a "slow" chopper, the word slow referring to the speed

of the neutrons, and was used for neutrons up to

eV. Fast choppers have also

been developed for use with neutron energy up to the keV range,50-52 with the

rotating cylinders of the chopper having different design, depending on the

requirements of the measurement.

The most important characteristic of any chopper is the width of the

neutron burst. In all choppers, the shape of the pulse is essentially triangular

with the base of the triangle being inversely proportional to the rotating speed

of the shutter. The shape of the pulse changes slightly with the neutron speed

and the shape of the channel. The width of the channel, which also affects the

pulse, is a compromise between acceptable time resolution and adequate count-

ing rate. Using choppers, neutron bursts with widths as low as

0.5 ps have been

a~hieved.~' The time resolution due to such a width is adequate for energies up

keV. At higher energies, ion beams from accelerators are used to provide

the neutron burst.

14.8.2

Pulsed-Ion

Beams

Narrow and intense bursts of neutrons for TOF experiments are obtained by

using ion beams. The ions are accelerated, strike a target, and produce neutrons

through a (charged particle, n)-type reaction. Examples of such reactions are

Neutrons produced by these reactions are in the MeV range.

510

MEASUREMENT

AND

DETECTION

RADIATION

The first accelerator to be used for neutron production was the cyclotron.53

Since that time, other types of accelerators have been utilized in TOF experi-

ment~~~-~~ such as electron linear accelerators; the Los Alamos Meson Physics

Facility (LAMPF), which accelerates protons to

800

M~v~~.~~; and the ORELA

facility at Oak Ridge, ~enn.~' The width of neutron bursts produced by

accelerators can be lower than

100

ps.61,62 If the burst becomes as narrow as

ps, the resolution is limited by the time response of the neutron detector.

14.9

COMPENSATED ION CHAMBERS

Neutron counters located close to a reactor core are subjected to both neutron

and gamma bombardment. Although a neutron counter-e.g., a

'OB

counter-is

mainly sensitive to neutrons, it responds to gammas too. At low reactor power,

when the neutron

flux

is small, the neutron signal is overshadowed by a signal

due to gammas emitted from fission products that had been accumulated from

earlier reactor operation. To eliminate the effect of the gammas, a compensated

ion

chamber

is used.

Compensated ion chambers operate in such a way that the gamma signal is

subtracted from the total

y) signal and the output is proportional to the

neutron signal only. Figure 14.24 shows the basic principle of a compensated ion

chamber. The counter consists of two compartments. One, coated with boron, is

sensitive to both neutron and gammas and produces a signal proportional to the

total radiation field. The other is sensitive to gammas only and produces a signal

proportional to

y radiation only. As Fig. 14.24 shows, the circuitry is such that

the

y signal is subtracted from the (y

n) signal, thus giving a signal propor-

tional to the neutron field only. The signal, in the form of a current, is measured

by a picoammeter.

Volume sensitive to

neutrons

gammas

Boron coat

Volume sensitive to

gammas only

Figure

14.24

compensated ion chamber.

NEUTRON

DETECTION

AND

SPECTROSCOPY

511

Collector

nsulator

Emitter

Figure

14.25

Configuration of a self-powered detector.

Correct compensation is achieved when the signal is zero in a pure gamma

field. This is accomplished by using the proper combination of volumes for the

two compartments or by changing the voltages or by a combination of voltage

and volume change. Typical compensation voltages

(V,)

are of the order of

the positive voltage V is of the order of

+800

Without compensation, a

detector of this type has a useful range from 2

10' to 2

lo4

neutrons/(m2

With compensation, the useful range is extended downward by about two

orders of magnitude. The sensitivity of compensated ion chambers is of the

order of 10-l8 ~/[neutrons/(m'

$1.

14.10

SELF-POWERED NEUTRON DETECTORS (SPND)

Self-powered detectors, as their name implies, operate without an externally

applied voltage. The incident radiation (neutrons or gammas or both) generates

a signal in the form of a current proportional to the bombarding

flux.

The

detectors are usually constructed in coaxial configuration (Fig. 14.25). The

central conductor is called the emitter and is the material responsible for the

generation of the signal. The outer conductor, called the collector, is separated

from the emitter by an insulator. The collector is made of inconel alloy, and has

the form of a metallic sheath encasing the insulator and the emitter.

The principle of signal generation in a self-powered detector is simple.

result of bombardment by radiation, the emitter releases electrons (betas) that

escape to the insulator and leave the emitter positively charged. If the emitter is

connected to the collector through a resistor (Fig. 14.251, current flows, which

512

MEASUREMENT

AND

DETECTION

RADIATION

when measured, gives an estimate of the incident

flux.

Note that this is not an

emitter-collector system: any beta particle escaping from the emitter contributes

to the current, regardless of whether or not it reaches the collector.

Because self-powered detectors have been developed for use inside the core

of power reactors, they are designed to have small size (a few millimeters in

diameter), to be able to operate for rather long periods of time (years) in the

intense radiation field of the reactor core without appreciable deterioration in

performance, and finally, to operate without an external power supply.

The performance of a self-powered detector is given in terms of its sensitiv-

ity

defined by the equation

where I(t)

detector current after exposure to the

flux

for time t

neutron

flux

Thus, the sensitivity represents the change in detector current per unit change

in the

flux.

Many elements have been considered as emitters for self-powered detec-

tor~.~~-~~ The ideal emitter should be such that the detector has

High sensitivity

2. Low burnup rate

Prompt response

4. Sensitivity to neutrons only

The material properties that determine these characteristics are discussed in

Secs. 14.10.1 and 14.10.2, after the equations for the detector current and

sensitivity are derived.

The properties of the insulator are also important. The insulator must have

a resistance of about

1012 ohms at room temperature and

lo9

ohms at reactor

operating temperature. The two insulators commonly used are magnesium oxide

(MgO) and aluminum oxide (Al,O,). Experiments have shown6" that the resis-

tance of MgO decreases with exposure to radiation, while that of A1203 does

not change. For this reason,

A1203 is gradually replacing MgO as an insulator

for self-powered detectors.

The self-powered neutron detectors are divided into those with delayed

response and those with prompt response. The characteristics of these types of

self-powered detectors are presented in Secs. 14.10.1 and 14.10.2.

14.10.1

SPNDs

with

Delayed Response

Rhodium, vanadium, cobalt, and molybdenum have been used as emitters for

SPNDs. Since rhodium SPNDs are the main in-core instruments for the deter-

mination of power distribution in pressurized-water reactors (PWR), they are

discussed first and in greater detail than the others.

NEUTRON

DETECTION

AND

SPEmROSCOPY

513

The si nal of rhodium SPNDs is produced as a result of activation of the

emitter

(

Rh) by the incident neutrons, and subsequent decay of the isotope

104

Rh that is produced. The decay scheme of lo4~h is shown in Fig. 14.26.

isomeric state with a 4.4-min half-life is produced with a 12-b cross section. The

ground state of lo4~h has a 43-s half-life and is formed with a cross section of

138

b (with thermal neutrons). It decays to '04pd with a maximum

P-

energy of

2.5 MeV. This decay, which takes place 98.5 percent of the time, is primarily

responsible for the signal of the rhodium detector. The isomeric state with the

4.4-min half-life contributes very little to the signal, but is responsible for a

residual current after reactor shutdown.

To identify the factors that improve sensitivity of the detector and lengthen

its life, one should look at the processes responsible for the generation of the

detector signal. This is done below, and equations for current and sensitivity are

derived for any emitter material.

Consider an emitter with an average neutron absorption cross section

exposed to a total neutron flux 4, and upon absorption of a neutron becoming

radioactive with a half-life T [or decay constant

(ln2)/T]. The number of

radioactive atoms N(t) present after exposure for time t is (see Eq. 14.16)

where

number of emitter atoms at

absorption cross section of emitter

cross section that leads to the state that contributes to the signal

self-shielding factor

The self-shielding factors corrects for the fact that the target (emitter) is thick,

as a result of which the

flux

in the emitter is depressed. Thus, interior atoms are

"shielded" from exposure to the full

flux

by the atoms close to the surface. The

2.5

MeV

Figure

14.26

The

decay

scheme

lo4~h.

514

MEASUREMENT

AND

DETECTION

RADIATION

0.25 0.5 0.75 1.0 1.25

Emitter diameter, mm

Figure

14.27

The

self-shielding factors for

rhodium detectors with 10-mil (0.254-mm) thick

MgO insulator (from Ref.

68).

shielding factor is less than

and decreases as the diameter of the emitter

increases (see Fig.

14.27).

If every decay of the radioisotope releases a particle

with charge

the current at time

is equal to

where

is a constant that takes into account such effects as self-absorption of

betas in the emitter or loss of betas in the insulator (see Fig.

14.27).

The factor

ua4/A

in the denominator can be neglected because, for all emitters of interest,

aac$/A

The exponential factors of

Eq.

14.62

have the following meaning.

The factor exp(-

At)

gives the response of the detectors. If the

flux

undergoes a step increase, as shown in Fig.

14.28,

the signal will rise exponen-

tially to its saturated value. If the

flux

goes down suddenly, the signal will decay

NEUTRON

DETECTION

AND

SPECTROSCOPY

515

again exponentially. The speed of response is determined by the half-life of the

isotope involved. Rhodium, with a half-life of

s, reaches saturation after

about 5 min. Vanadium (52~), with a half-life equal to 3.76 min, reaches

saturation after about 25 min.

The factor

exp(-sua+t) gives the burnup rate of the emitter. It is the

factor that determines the lifetime of the detector, because, as seen below, the

decrease in sensitivity with time is essentially given by

this

same factor.

Assuming saturation, the sensitivity of the detector is given (using Eqs. 14.60

and 14.62) by

If the emitter diameter is

its length is L, its density is

and its atomic

weight is A, then the number of atoms is

(

p~D2/4XLNA/A), where NA

is Avogadro's number.

Substituting into Eq. 14.63, one obtains an equation for the sensitivity per

unit length:

Example

14.6

What is the sensitivity of Rh detectors 0.5 mm in diameter,

per unit detector length under saturation conditions, for a new detector?

Answer

For rhodium,

139 b,

150 b,

12.4

lo3

kg/m3, A

103, and

1.602

10-19

At the beginning of life (t

01,

3.17

10-23ks (A/m)/[neutrons/(m2. s)]

Typical values of

are about 0.4.68 Since the

flux

in a large power reactor (1000

MWe) is about

loi7

neutrons/(m2

s)[1013neutrons/(cm2

s)] at full power, and

the typical detector has a length of about 0.10 m, the expected current is of the

order of

(0.8M0.8

l~-~~)(A/m)/[neutrons/(rn~

s)1[1017 neutrons/(m2

s)](0.1 m)

1.92

200nA

Equation 14.64 shows that to achieve high sensitivity, one should select an

emitter with high cross section

ue and large diameter

The diameter affects

516

MEASUREMENT AND DETECTION

RADIATION

the sensitivity through the factor

and through the shielding factors

and

which decreaset as the diameter increases. The net result is that the sensitivity

changes roughly as the first power of the diameter.68

high cross section increases the sensitivity but also increases the rate at

which the sensitivity decreases with time. Indeed, from Eq. 14.64, the ratio of

the sensitivity after exposure for time

to its value at time

Table 14.6 gives the characteristics of several self-powered detectors. Rhodium

detectors have the best sensitivity but also the largest burnup rate. Their change

of sensitivity with time is important and necessitates a correction before the

signal is used for the determination of power. The correction is not trivial

because of changes in self-absorption effects in the emitter, and may introduce

errors unless the detector is calibrated properly.

Despite the drawback of large burnup, rhodium SPNDs are used extensively

in nuclear power plants, especially in PWRs for the determination of power

distribution, fuel burnup, and other information related to the performance of

the core. The detectors are inserted into a certain number of "instrumented"

fuel assemblies through guide tubes. Every instrumented assembly has seven

equally spaced SPNDs (a background detector and a thermocouple are also

included in the package; see Fig. 14.29) for the measurement of the

flux

at seven

axial locations. The outputs of the detectors, corrected for background, are

transmitted to the plant computer, where after appropriate corrections are

applied, the power, fuel burnup, plutonium production, etc., are calculated.

Every PWR has a least

instrumented assemblies, which means that the

flux

monitored at more than

350

locations.

h he

factor

is not constant over prolonged exposure. It tends to increase as the emitter

bumup continues because a smaller number of emitter atoms is left for self-shielding.65

Table

14.6

Characteristics of Self-Powered Detectors with 0.5-mm

Emitter ~iarneter~~.

Sensitivity ~urnupt/year

(%)

Emitter

(A/m)/[2

lot7

neutrons/(ma .s)]

for

10'' neutrons/ (ma as)

Response

2.4

lo-=

Delayed

1.5

lo-1

0.3 Delayed

Co 3.4

lo-*

2.3 Prompt

Mo 1.7

lo-'

0.9 Prompt

2.6

lo-"

0.2 Prompt

NEUTRON

DETECTION

AND

SPECTROSCOPY

517

Thermocouple

Background detector

Yros section

wall of guide tube

Figure

14.29

Each detector tube contains seven SPNDs, one background detector, and one thermo-

couple.

14.10.2

SPNDs with Prompt Response

Neutron-sensitive self-powered detectors with prompt response operate on a

different principle than rhodium and vanadium SPNDs. The emitter, in this

case, absorbs a neutron and emits gammas at the time of capture. It is these

capture gammas that are responsible for the signal, and since they are only

emitted at the time of the neutron capture, the detector response is instanta-

Collector