Tsoulfanidis N. Measurement and detection of radiation

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398

MEASUREMENT

AND DETECTION OF RADIATION

12.5

DETECTION OF GAMMAS

WITH

AN NE

213

ORGANIC SCINTILLATOR

The NE 213 organic scintillator has emerged as one of the leading fast neutron

spectrometers (see Chap. 14). As a gamma spectrometer, the NE 213 scintillator

has an efficiency lower than that of

NaI(T1) and an energy resolution that is

poor compared to that of semiconductor detectors. There are certain applica-

tions, however, where high-energy resolution is not the most important factor.

One such application is detection of gammas in a mixed neutron-gamma field.

There, the ability of the NE 213 scintillator to discriminate against neutrons

makes it an attractive gamma detector.

Because gammas are detected by the NE 213 scintillator mainly through

Compton interactions, the response function of the detector consists of the

Compton continuum. The response function has been calculated6 and mea-

sured' for several gamma energies. Figure 12.19 shows a comparison of calcu-

lated and measured response functions.

To obtain the gamma spectrum from the source, the measured NE 213

spectrum must be unfolded. Results of matrix-inversion unfolding applied to the

measured spectrum of 24~a are shown in Figs. 12.20 and 12.21. The spectrum

shown in Fig. 12.21 was obtained by unfolding the spectrum of Fig. 12.20 with

the code

FORIST,~ which is a variation of the code FERDOR.~ Both codes use

Figure

12.19

Comparison of measured

(point) and calculated (line)

213

response functions for 2.1-,

4.0-,

and

Pulse height, light units

6.11~e gamma rays (from Ref.

7).

PHOTON

(GAMMA-RAY

AND

X-RAY)

SPECTROSCOPY

399

Figure

12.20

Gamma-ray pulse height spec-

trum produced by

213 for

24~a source.

The two Compton edges are due to the 1.37-

Pulse

height,

light

units

and 0.75-MeV gammas (from Ref. 7).

the least-squares unfolding method described in Sec. 11.5.4. Another program

used for gamma spectra analysis is called

SAMPO,

adapted for use with

microcomputers.10

12.6

DETECTION OF

X-RAYS

WITH

PROPORTIONAL COUNTER

Gas-filled chambers operating in the proportional counter range are used in

certain special applications for the detection of X-rays with energy less than 100

Figure 12.21

The spectrum of Fig. 12.20, un-

folded with the code FORIST. The vertical

error bars show

1 standard error due to

Gamma-ray

energy,

MeV

counting statistics (from Ref. 7).

400

MEASUREMENT

AND

DETECTION

RADIATION

keV. At this energy range, the photons interact only through the photoelectric

effect. Since the photoelectric cross section increases as

Zm,

with

3-5, it is

important to have a window made of very low-Z material and a gas with as high

a Z value as possible.

X-ray proportional counters are usually cylindrical with a very thin beryl-

lium window located either on the side or at the front end. They use a gas that is

a mixture of a noble gas-He, Ne,

Ar,

Kr,

Xe-with methane, at a pressure of

about

atm.

The energy resolution of these counters is such that the

FWHM

is 1-2 keV

at 20 keV. Thus, proportional counters are superior to scintillation counters in

this energy range.

12.7

DETECTION OF GAMMAS

WITH

Ge DETECTORS

As mentioned in Sec. 7.5.5, the Ge(Li) detectors have been replaced by Ge

detectors, which are devices that use hyper pure germanium (impurity concen-

tration

1016

atoms/m3 or less). The main advantage of

over Ge(Li) detectors

is that the former should be kept at low temperatures only when in use; the

latter must be kept cool at all times.

The cooling of Ge detectors is achieved by permanently mounting the

detector on a cryostat. The cryostat consists of a reservoir or Dewar containing

the cooling medium and a vacuum chamber housing the detector. The Dewar is

made of two concentric metal containers (Fig. 12.22) with the space between the

two containers evacuated for thermal insulation. In one design, called the

"dipstick" (Fig 12.22a), the detector is housed in a separate vacuum chamber

and the cooling rod is made of copper. The cooling medium is usually liquid

Detector

Dewar

Chamber

liquid nitrogen

r--.

_---

C---

Figure

12.22

(a)

dipstick cryostat (cross section).

(b)

integral cryostat (cross section) (courtesy

of Canberra Nuclear).

nitrogen (boiling temperature

196"C, or

K).

In another design, called the

integral cryostat (Fig. 12.226), there is a common vacuum chamber for both the

Dewar and the detector. One version of the integral cryostat is provided with a

rotary vacuum seal, which allows the detector chamber to be rotated

180".

With

respect to cooling, one manufacturer (Canberra) has designed and is offering a

cryoelectric cryostat that uses a commercial refrigerator with helium gas as the

refrigerant.

The vacuum chamber that contains the detector is made of stainless steel.

The chamber protects the detector from dirt and, by being evacuated, prevents

condensation of vapor on the detector surface or electrical discharge when high

voltage is applied to the detector.

metal envelope, with a very thin window at

its end for the passage of the incident photons, surrounds the detector. The

window is made of beryllium, aluminum, or a carbon composite fiber. Transmis-

sion characteristics of several window thicknesses are shown in Fig. 12.23. Most

commercial cryostats include the preamplifier as a standard component.

Reduction of background in any measurement is very important. It becomes

absolutely necessary in cases when the sample to be counted is a very weak

radiation source and its activity barely exceeds the background. Complete

elimination of the background radiation is impossible, but reduction of it is

feasible by using special shields. Common shields are made of lead or steel and

are

0.10-0.15

m thick. Figure

12.24

shows a typical arrangement of the cryostat,

the detector, and the shield. Photographs of two commercial detectors, cryostats,

and multichannel analyzers are shown in Fig.

12.25.

12.7.1

Efficiency

Ge Detectors

The efficiency of Ge detectors quoted in the list of specifications by the

manufacturer may be a relative full-energy peak efficiency or an absolute

Energy (keV)

Figure

12.23

Transmission characteristics of various detector windows. The thicknesses shown are in

mils of inches (courtesy of Canberra Nuclear).

402

MEASUREMENT

AND

DETECTION

RADIATION

Shield

Detector

--*

Figure

12.24

typical arrangement of the detector,

the shield, and the cryostat.

efficiency. Relative efficiencies are referenced in terms of the absolute full-en-

ergy peak efficiency of a 76 mm

76 mm (3 in

3 in) NaI(T1) crystal. The

measurement (or calculation) is based on the 1.33-MeV peak of 60~o. It is

assumed that a

60Co source of known strength is positioned 0.25 m away from

the face of the detector. A count is taken for a period of time, and the absolute

full-energy peak efficiency of the Ge(Li) detector is determined by dividing the

total number of counts under the 1.33-MeV peak (shaded area, Fig. 12.26) by

the number of photons emitted from the source during the same time period.

This absolute efficiency is divided by 1.2

lop3, which is the absolute efficiency

of a 3 in

3 in NaI(T1) crystal 0.25 m from the source, to give the relative

efficiency quoted in the specifications.

Absolute efficiencies as a function of energy for four types of Ge detectors

are shown in Figs. 12.27-12.30. In Fig. 12.27, a Ge wafer is used to make what

the manufacturer calls a low-energy Ge

(LEGe) detector. In this detector, a p

contact is fabricated on the front face and the cylindrical surface with implanted

boron; on the rear face, an n

contact is formed with lithium diffused along a

spot that is smaller than the full rear area of the device. The efficiency of this

detector is dropping for energies below 5 keV because of absorption in the Be

window; at the other end of the graph, the efficiency drops for

100 keV

because of a corresponding decrease in the value of the total linear attenuation

coefficient of gamma rays in Ge (Fig. 12.31). A coaxial Ge detector and its

efficiency are shown in Fig. 12.28. The contacts of this detector are formed by

diffused lithium

(n contact) and by implanted boron (p contact). The diffused-

lithium n contact is given by the manufacturer as 0.5 mm thick.

variation of

the coaxial detector, called the reverse-electrode (REGe) detector and its

efficiency are shown in Fig. 12.29. In the REGe detector the electrodes are

opposite to those of the "normal" coaxial: the p-type electrode (formed by

ion-implanted boron) is on the outside, and the n-type contact (formed by

diffused lithium) is on the inside. This electrode arrangement leads to decreased

window thickness (the p contact may be as thin as 0.3 pm; the Be window is

0.5 mm), which, in turn, results in higher efficiency at lower energy (compare

efficiency curves of Figs. 12.28 and 12.29). Finally, in Fig. 12.30 a Ge well-type

PHOTON (GAMMA-RAY AND X-RAY) SPECTROSCOPY

403

Figure

12.25

(a)

Ge detector with its cryostat and multichannel analyzer (reproduced from Instru-

ments for Research and Applied Sciences by permission of

ORTEC, Oak Ridge, Ten-

nessee).

(b)

Portable Ge detector system with its cryostat and multichannel analyzer (courtesy of

Canberra Nuclear).

404

MEASUREMENT

AND

DETECTION

RADIATION

1.1

MeV

1.33

MeV

Figure

12.26

sketch of the 60Co spec-

trum, indicating how it is used for effi-

ciency and peak-to-Compton ratio deter-

Photon energy

mination.

detector is shown. The special characteristic of this device is its increased

efficiency due to its particular geometry. The solid angle approaches 47r,

resulting in close to 100 percent efficiency in a certain energy range.

For Ge detectors other than the well-type, the efficiency is low, relative to

Na(Tl) scintillation counters. This statement holds true for Si(Li) detectors as

well (see Sec. 12.9). Lower efficiency, however, is more than compensated for by

the better energy resolution of the semiconductor detector. Figure 12.32 illus-

trates the outstanding resolution characteristics of a semiconductor detector by

showing the same spectrum obtained with a NaI(T1) and a Ge(Li) detector.

Notice the tremendous difference in the FWHM. The

Ge(Li) gives a FWHM

1.9 keV, while the NaI(T1) gives FWHM

70 keV.

Consider a case of 10,000 counts being recorded by a 3 in

3 in Na(T1)

detector under the 1.33-MeV peak of 60Co.

Ge detector with 10 percent

relative efficiency will record only 1000 counts. The FWHM of the Na(T1) peak

is 70 keV; the corresponding width of the Ge peak is about 2 keV. Since the

total number of counts under the peak is proportional to the product of the

FWHM times the peak, the heights of the two peaks are related by

Height of Ge peak

- -

(1000/2) 70

Height of NaI(Tl) peak (10,000/70) 20

3.5

Thus, even though the Ge detector is only 10 percent efficient, relative to the

NaI(Tl) crystal, it produces a peak that is 3.5 times higher.

Another parameter specified by the manufacturer of Ge detectors is the

peak-to-Compton ratio

(PCR). Looking at Fig. 12.26, the PCR is defined by the

equation

height of 1.33-MeV peak

PCR

average height of Compton

plateau of 1.33-MeV peak

The average of the plateau is taken between 1040 and 1096 keV, in accordance

with IEEE Standard No.

325-1971." The PCR is important because it indicates

the capability of the detector to identify lowenergy peaks in the presence of

PHOTON (GAMMA-RAY AND X-RAY) SPECTROSCOPY

405

10.1

I.'

0.0

incident

radiation

# #

1/-

Be Window

Absorption Edge\

I \I

Energy (keV)

(b)

Figure

12.27

(a)

LEGe detector.

(b)

Absolute efficiency as

function of energy for a detector size

shown on the graph and 2.5-cm distance assumed between source and detector (from Canberra, Ref.

2).

406

MEASUREMENT AND DETECTION

RADIATION

Energy

(keV)

(b)

Figure 12.28

(a)

Coaxial Ge detector.

(b)

Absolute efficiency as a function of energy for a distance

2.5

cm between source and detector (from Canberra, Ref.

2).

PHOTON (GAMMA-RAY AND X-RAY) SPECTROSCOPY

407

incident radiation

Be Window

(0.5

mm)

N+

Contact

Energy (keV)

(b)

Figure 12.29

(a)

REGe coaxial detector. (b) Absolute efficiency as a function of energy for a

distance of

2.5

cm between source and detector (from Canberra, Ref.

2).