Double-slit experiment: What is the speed of the neutrons?

[kamhogo]

Homework Statement

An experiment was performed in which neutrons were shot through two slits spaced 0.10 mm apart and detected 3.5 m behind the slits. The following figure shows the detector output. Notice the 100 μm scale on the figure. To two significant figures, what was the speed of the neutrons?

Homework Equations

Δy = λL/d (spacing between maxima)
λ = h/mv (de Broglie λ)
m= 1.67*10^-27 Kg

The Attempt at a Solution

The spacing between the maxima appears to be 100*10^-6 m in the figure, so:
(Δy*d)/L = λ = ((100*10^-6 m)*(0.1*10^-3 m))/3.5 m = 2.85714*10^-9 m = de Broglie λ

v = h/(mλ) = (6.626*10^-34 J.s)/((1.67*10^-27 Kg)*(2.85714*10^-9 m) = 138.9 m/s

This answer is incorrect. Can someone please give me a hint as to what I'm doing wrong?

 

[BvU]

spacing between the maxima appears to be 100*10^-6 m in the figure

Could you check that once more for me ?

(oh, and 138.9 is not what I would call 'to two significant figures' ! )

 

[kamhogo]

Could you check that once more for me ?

(oh, and 138.9 is not what I would call 'to two significant figures' ! )

Hum, on second look it looks more like (2/3)(100*10^-6m).

Which gives Vneutron = 2.1*10^2 m/s; which is the correct answer! Thanks a lot!

 

[BvU]

well done!