SURFACE CONTAMINATION MEASUREMENTS AT KEK
H. Hirayama,s. Miura, s. Ban and K. Kondo

National Laboratory for High Energy Physics

Surface contamination measurements around a high-energy accelerator
cannot be done using a survey meter due to radionuclides induced in accelerator
structures. Smear method may be only one method that is applicable in thesecircumstances. In ordinay case, gross a-ray measurements are used in smear
method supposing one a-ray is emitted per decay. But around a high-energy
accelerator, this estimation causes underestimate because a main nuclide donot emitt a-ray at disintegration. At KEK, an effective detection efficiency
of a 2"<j>G-M tube has been determined from the comparison with y-ray measurements
and is used in a routine measurements. At first, a smeared sample is measured
with a Ge(Li) detector and a contamination of each nuclide is determined.
Next, the same sample is measured with 2"<1> diameter GM tube, and the effective
detection efficiency is determined from the comparison with the both results.
The contamination of each nuclide and the obtained efficiency for the several
case are shown in Table 1. Thus obtained effecincy of course varies with thevariation of nuclide composition. As an example, in Fig. 1, the variation ofthe detection efficiency at the floor of a slow extraction beam line (EP-2) isshown as a function of the time after beam-off. In this case, main nuclide
included are 7Be and 24Na, and the efficiency raidly · decreases as the decayof 24Na. In spite of the wide variation of nuclide composition at the different
place and the different time after beam-off, always 7Be is main nuclide. Theeffective detection efficiency become minimum when 7Be only is included in thesample. Therefore, the detection efficiency of 7Be, 3 %, is suitable for routine measurements.
Another important point is that there is a possibility a linear relation
between the surface contamination and the exposure dose rate at the surface,
Obtained results for Main Ring duct surface are shown in Fig, 2. The dispersion is larger wide, but the linear relation can be seen as a whole. The
value of 2.9 x 10-4 and 8.0 x lo-5 ]JCi/cm2 per R/h at surface can be 'obtained
as the least upper limit for the Main Ring duct (SUS) surface and the EP-2duct (AL) surface or the floor, respectively. These values are useful as astandard of the surface contamination control.

..
-EP-2 Floor
107 Be/Total Activity

Fig. 1.

50o!:-·-----=s*-o~----1=*oo-::------.,.,!150~,-_ ____.oTime after Accelerator Stop,hour-81-

Variation of the GM
Tube Detection
Efficiency with theTime after AcceleratorStop.

Table 1

Surface Contaminati on of Individual Nuclide andG-M Tube Detection Efficiency

PlaceDate and Timeafter beam off7 Be

Main Ring Duct

Main Ring Duct

Main Ring Floor

EP-2 Duct

EP-2 Floor

1979. B.
(30 day)

1980. 6.
(1.5 day)1980. 6.
(2 day)1980. 6.
(l day)1980. 6.
( 1 day)

306 dps/100crn 237.9 dps/100cm

23.1 dps/120
em

15 .~ dps/100
cm2

0. 001~

0. 0005

3. 4 7

0. 551

2

5. 92 dps/lOOcm

2

1eF
22Na

1~ .1

2 "Na

lfljmsc

2.08

4 4 Sc
Lj 6

0.19

Sc

24.1

0.18

31.6

0. ~1106l. 05

It 7 Sc

0.11"evS) Cr

52Mn

54Mn

2. 92

0. 30

40.1

0.180. 063

S6Hn0.0058

56 Co

6. 47

57 Co

~. 45

0.029

seCo

0. 072Total

539 dps/100cm2

GM TubeCounting Rates

42.5 dps/100crn2

5. 98 dps /100cm2

26.6dps/~~2

16.0 dps/~~327 cps

l. 23 cps

0. 222 cps

2. 21 cps

0. 893 cps

5. 0 %

2. 9 ,;

3. 7 %

8. 3 %

5. 6 %GM TubeDetection
Efficiency

Accelerator Vacwm Tube Surface•...

10-4..
.. .
.·.

Fig. 2.
Relation between Surface
Contaminati on and Exposure
Dose Rate at Surface·. ..

•10'mR/h at Surface-8 2-