I'm writing a code about radiation transportation and have generated isotropic vectors as shown by my code. I'm now unsure about how to generate isotropic steps with lengths distributed as exp(-x/lambda). Any suggestions?

```
def isotropic_unit_vectors():
nparticles = 1000
lambda_a = 45
lambda_s = 0.3

sigma_a = 1/lambda_a
sigma_s = 1/lambda_s
sigma_T = sigma_a + sigma_s

lambda_T = 1/sigma_T
u = np.random.uniform(low = 0, high =1, size = 1000)
step = -lambda_T*np.log(u)
theta = 2*np.pi*np.random.rand(nparticles)
phi = np.arccos(2*np.random.rand(nparticles)-1)

dx = step*np.cos(theta)*np.cos(phi)
dy = step*np.cos(theta)*np.sin(phi)
dz = step*np.sin(theta)

rand = np.column_stack((dx,dy,dz))
fig = plt.figure()
ax = fig.add_subplot(111, projection='3d')
ax.scatter(dx, dy, dz, c='r', marker='.')

ax.set_xlabel('X Label')
ax.set_ylabel('Y Label')
ax.set_zlabel('Z Label')
plt.show()
return rand

```

edit: I want multiple particles to start at 0,0,0 then move randomly.