Measurement techniques for characterizing and using low background germanium detectors

412

Nuclear Instruments and Methods in Phystcs Research 223 {t'984~ 412 41 ? North-llolland AFnMerdao~

MEASUREMENT TECHNIQUES GERMANIUM DETECTORS W i l l i a m H. Z I M M E R

FOR

CHARACTERIZING

AND

USING

LOW

BACKGROUND

a n d S a n f o r d E. W A G N E R

EG&G ORTEC. 100 Midland Road. Oak Ridge, Tennessee 37830, USA

An investigation has been undertaken to determine whether an order of magnitude background reduction from present typical cryostat-detector systems can be obtained through the use of low background components, in order to measure progress in this task. a standard, ten-centimeter lead shield was fitted with a five-centimeter, oxygen-free high-conductivit3 copper liner and a borated polyethylene neutron absorber, This reduced the contribution of uranium-238, thorium daughters, and radium daugthers from the shield as seen by the detector by 1.3.0.02. and 0.1 Bq respectively. The methodology of determining very low net photon peak areas m the presence of high continuum levels to assure maximum accuracy was verified and ~s presented, By these means the background activities of detectors are being measured at the - 10 z Bq per nuclide level and detector component materials at the Bq per gram level. both with total uncertainties of less than 50% lo. The hardware and software developed is being used to meas,ure the background activity of the detectors and for the analysis of low activity samples.

!. Introduction A n a t t e m p t is being made to reduce high-purity g e r m a n i u m detector b a c k g r o u n d b y an order of magnitude through materials selection. The m e a s u r e m e n t m e t h o d s d e v e l o p e d involve e s t a b l i s h i n g c o u n t i n g p a r a m e t e r s a n d techniques to (1) define initial detector b a c k g r o u n d activities (starting point), (2) normalize the assay of test c o m p o n e n t materials, a n d (3) provide a m e a s u r e m e n t of progress, Detector b a c k g r o u n d consists of the radiation from detector c o m p o n e n t s that is recorded as full-energy p h o t o p e a k s a n d the associated c o n t i n u u m from these photopeaks. Also included in detector b a c k g r o u n d is the pure noise or c o n t i n u u m from cosmic a n d other very high-energy i n t e r a c t m n s occurring outside the detector. The former can be controlled by materials selection while the latter cannot. Therefore. the initial and progress m e a s u r e m e n t s were related only to the rate of p h o t o p e a k f o r m a t i o n in the detector.

2. Shield The first task was the isolation of the detector from all sources of peaked b a c k g r o u n d radiation o t h e r than its o w n c o m p o n e n t s . The two principal sources of this radiation are the shield a n d the earth. Fig. 1 shows the present configuration of the low b a c k g r o u n d measurem e n t shield. A 5 cm inner shield linear of low background, oxygen-free high-conductivity copper was placed a r o u n d the detector to act as a passive shield. 2.5 cm of 5% b o r a t e d polyethylene was placed on the 0 1 6 7 - 5 0 8 7 / 8 4 / $ 0 3 . 0 0 © Elsevier Science Publishers B.V. ~ N o r t h - H o l l a n d Physics Publishing Division)

outside b o t t o m of the shield to reduce any neutrx~ flux a n d to minimize the reaction with c a d m i u m grading. The boil-off of nitrogen from the delector dewar is vented to the space between the liner a n d the detector to displace atmospheric radon These measures reduced the concentration of uranium-238, t h o r i u m daughters. a n d r a d i u m daughters from the shield as seen by tile detector through or from a commercial 10 cm lead shield by 1.3.0.02. a n d 0.1 Bq respectively.

3. A definition of the starting point A 30~ relative efficiency, reverse electrode, n-type coaxial high-purity g e r m a n i u m detector was chosen as the test vehicle for gauging the effects of c o m p o n e n t c o n t r i b u t i o n s to detector background. Table t is t h e 200000 s assay of this detector in the b a c k g r o u n d m e a s u r e m e n t s shield, T h e values listed were defined as the starting point for further improvements, The units of activity are disintegrattons per day. They are o b t a i n e d by normalizing the spectral data to the efficiency for a point source on the end cap of the detector. This convention was c h o s e n as insurance against loss of the trial detector during the tests a n d as a first order means of extrapolating the data generated to different size detectors, The alternative is the reporting of the peak data unnormalized for efficiency, These results are specific to a single detector, a n d they are reported in table 1 with units o f g a m m a s per day. Both m e t h o d s of reporting are being used. A n o t h e r c o n v e n t i o n that had to be a d o p t e d was the

413

W. tt. Zimmer, S.E. Wagner / Low background germanium detectors

cm OFHC Copper

2.5 cm 5% Borated Polyethylene

-~---[

20 in. diam

Fig. 1. Low background detector shield.

s e p a r a t e d e t e r m i n a t i o n of activity for e a c h p h o t o p e a k e n e r g y r e c o r d e d . E v e n a c u r s o r y look at table 1 indic a t e s little c o r r e l a t i o n in activity for d i f f e r e n t p e a k s o f t h e s a m e nuclide. T h e d i s t r i b u t i o n o f e m i t t e r s in the

c o m p o n e n t s of t h e d e t e c t o r is n o t p r e d i c t a b l e , n o r is the p a t h l e n g t h for a t t e n u a t i o n in t r a n s i t to the detector. N o assumption of equilibrium among the primordial decay daughters has been made.

Table 1 A 200000 s assay of background for a 30% relative efficiency, HPGe detector in the background measurements shield and the starting point for gauging subsequent background improvements through detector component replacement Energy (keV)

Activity (disintegr./day)

Activity (gammas/day)

Per cent uncertainty (counting lo)

Per cent uncertainty (total lo )

46.5 63.3 92.6 185.7 238.6 583. l 727.2 766.6 911 1001 1460.8

2.495E + 03 1.253E + 03 1.009E + 03 1.451 E + 03 5.375 E + 03 2.705E + 03 1.128E + (13 1.249E + 03 1.361E+03 1.413E + 03 3.870E + 03

9.171E + 02 4.610E + 02 3.226E + 02 2.873 E + 02 9.472E + 02 2.242E + 02 7.150E + 01 7.394E + 01 6.350E + 01 5.741 E + 01 8.510E + 01

4.3 8.0 10.3 14.6 5.1 9.2 26.2 24.8 21.5 21.4 13.2

5.8 10.7 12.4 15.1 6.5 10.0 26.5 25.2 21.8 21 .~ 13.7

I11. S P E C T R O M E T R Y

414

14".H. Zimmer, S.E. Wagner

4. Determination of low level activities Detector c o m p o n e n t materials are assayed in either of two geometric configurations; a large diameter disc having a thickness of approximately l cm. or a 0.5 I. IEEE Standard Marinelli beaker [1]. The background spectral data presented in table 1 was re-evaluated using the above calibrations and applied to the assa~ of c o m p o n e n t activities as a peak background correction. That is, the activity of each nuclide in the background spectrum was determined from a net peak area in the spectrum and was re-interpreted in the sample spectra as a reduction in the net p h o t o n spectral peak area and a corresponding increase in background. Attenuation corrections were also applied to the assay of detector c o m p o n e n t activities using photon cross sections from Storm and Israel [2]. After the individual peak background corrections had been applied, the attenuation corrections caused all of the detectable net peaks for each sample nuclide to be calculated with the same activity within the limits of uncertainty. The calculation results verify the methodology and permit a more accurate assessment of the available data. Table 2 illustrates, in a limited way, the results of these calculations. Net count (area) determination methods at very low activity levels were tested. In general, methods that subtract a s m o o t h e d background continuum from sample spectra miss low yield peaks, and as a consequence. bias background high and net peak areas low. Methods that determine u n s m o o t h e d minima, higher and lower in energy than each peak, underestimate the background

L.owbackground germantum detector.~ and overestimate the net area. Of the tv, o methods, the atter is the more conservative, and the level of bias introduced diminishes with the length of spectral acquisition time. All data acquisitions m these studies arc at least 200000 - in duration. 1he unsmoothed minima method of background determination was used m these studies. Background continuum was determined_ at locations higher and lower in energy than each peak, as the five consecutive channels with the Iowe~t con> bined channel content. A1 0.5 keV per channel in 4096 channels, three consecunve channels proved an inadequate sampling of the data while seven or more consecutive channels tended to again bias background continuum high. Net peak area was the integrated channel content above the background continuum slope. No smoothing was performed and no peak-shape fitting was performed except to resolve muhiplets. Repetitive background and sample acquisitions confirmed that the net peak areas, and consequently, the activities, are reproducible within the derived counting statisuc~. Low background detector c o m p o n e n t actwities were reported in units of disintegrations per minute per gram. Table 2 lists five representative assay, of billets of low activity magnesium. Activity values preceded by "" < are M i n i m u m Detectable Activity I M D A ) [3] values calculated with a "'C" value of 2 t50f~ counting uncertainty at the 67% confidence leveb. "[hese assays were performed on the same detector but during various phases of its background reduction, The decreasing M D A s reflect decreases b a c k g r o u n d s All of the calculations and corrections in this secuon

'Fable 2 Assay of five billets of low activity magnesium. Each column represents an assay performed later in time. ~o the ~rnnimum detectable activities reflect progress in reducing the background activity of the test detector Nuclide

Energy (keV)

228Ac

911 969 2615 238 583 63 1001 767 609 1764 1120 46.5 186 144 1461 1173 1333 662

22STh

238U

214Bi

21°pb 235U 4°K 6°Co

137Cs

Activity (d/m/g)

Activity (d/m/g)

Actiwt~, (d/m/g)

Actwity (d/m/g)

< 7.4E--02

< 4.7E-02

( 2.5E-02

- 3.6E--02

< 4.5E - 02

< 2.5E - 02

< 2.9E - 02

< 2.2E-01

< 2.3E-01


< 8.4E-02

2.912E- 02 ~.792E- 02 3.003E- 02 - 8.8E-02

< 6.1E-02

< 3.9E-02

< 2.7E-02

< 2.2E-02

< 1.7E-02

< 6.6E-01 < 6.9E-04

< 1.8E - 01 < 2.8E-02

< 1 3E-O1 2.265E-02

< 2.5E- 01 < 3.1E-02

< 1.2E- 01 < 1.7E--02

< 1.1E-0I < 1.1E-02

< 9.5E- 02 < 8.7E-03

6.982E-02 < 7.1E-- 03

< 2.8E-02

< 2.2E- 02

< 1.5E- 02

< 1.0E~02

~ 8.5E-03

9.718E-02 1.003E- 01 8.809E - 02 7~711 E - 02

3.049E - 01 1.723E-02

ActLvitv id / m / g )

< 8.6E-02 < 1.4E-02

W.H. Zirnmer, S.E. Wagner / Low background germanium detectors

415

Table 3 Initial and current background for the 30% relative efficiency test detector. The current activity and uncertainty values are the result of a 400000 s assay F_ncrgy (kc~, I

Initial activity (gammas/day)

Current activity (gammas/day)

Per cent uncertainty (counting la )

Per cent uncertainty (total la }

46.5 63.3 92.6 185.7 238.6 295.2 351.9 583.1 609.3 661.6 727.2 766.6 911 969 1001 1120.3 1173 1238 1332.5 1460.8 1764.5

9.171E+02 4.610E+02 3.226E + 02 2.873E + 02 9.472E + 02

1.229E+02 1.121E+02 1.144E + 02 1.013E + 02 5.404E + 01

16.3 18.6 13.8 23.3 47.3

16.7 19.9 15.4 23.6 47.4

3.233E+01

45.6

45.8

3.665 E + 01

34.1

34.3

2.419E+01

33.0

33.2

2.242E + 02

7.150E+01 7.394E + 01 6.350E+01 5.741 E + 01

8.510E+01

are contained in the standard E G & G O R T E C G E L I G A M T M application software.

5. Detector background reduction At this time a large variety of c o m p o n e n t materials have been tested. M a n y of the lower activity materials have been fabricated into detector c o m p o n e n t s and used to replace earlier c o m p o n e n t s in the test detector. This is a continuing process of identifying and using lower activity c o m p o n e n t materials, building an inventory of these materials, a n d verifying the quality of a source of supply once established. Table 3 restates the initial activities in units of g a m m a s per day for the test detector and lists the current activities. All of the peaks encountered in comp o n e n t materials and used in the analyses are also listed. The current test detector values are the result of a 400000 s data acquisition, twice as long as the initial acquisition. The goal of a ten-fold reduction has not been met as yet. However, there are currently no detectable photopeaks higher in energy than the 609.3 keV peak except at 1173 keV. The s u m m a t i o n s of net peak g a m m a s per day have been reduced in the test detector from 3511 and 598 a n d for peaks with activities higher than 100 keV, from 1810 to 249. These results are encouraging, and they invite the conclusion that a tenfold reduction in peaked activity is attainable.

6. Conclusions The project to establish counting techniques that would define low b a c k g r o u n d detector activities and allow the use of such detectors to assay low activity materials has been accomplished. The d e v e l o p m e n t of meaningful nomenclature for background sources and activities was no less i m p o r t a n t than the development of the spectrometric capabilities. These capabilities include the reproducible d e t e r m i n a t i o n of small net photopeak areas, the assay of the activity due to each b a c k g r o u n d photopeak, a n d the application of these activities individual as peaked b a c k g r o u n d corrections. Using these techniques in conjunction with a t t e n u a t i o n corrections, the assay of average sample nuclide activities is the same as the activities calculated for the detectable peaks of each nuclide. This is a verification of the validity of the techniques a n d makes o p t i m u m use of scanty data.

References [1] ANSI/IEEE Standard 680-1978, IEEE Standard Techniques for Determination of Germanium Semiconductor Detector Gamma-Ray Efficiencies Using a Standard Marinelli (Reentrant) Beaker Geometry (Inst. of Electrical and Electronics Engineers, Inc., New York, 1978). [2] E. Storm and H.I. Israel, Nuclear Data Tables, 7 (6) (Academic Press, New York, 1970). [3] J.A. Cooper, Nucl. Instr. and Meth. 82 (1970) 273. Ill. SPECTROMETRY